How can I tell the difference between unmarked sugar and stevia?

Don't throw it away. Worst case you use up the mystery sweetener on yourself, and use a new pack of Stevia for your wife and anything you share. That's what I recommend if you're not convinced by my solution or don't have sensitive kitchen scales.

At room temperature, sucrose (normal sugar) is very soluble in water: about 200 g of sugar will dissolve in 100 g of water.

It's harder to get a figure for the solubility of Stevia as it's not a single compound but a mixture of related compounds. However various patents claim "high solubility" stevia preparations of around 30 g per 100 g water, or around 1/7 as much.

This is enough of a difference to test: If (with plenty of stirring but no heat) the mystery sweetener will dissolve in an equal mass (weight) of water, it's sugar. If not it's stevia. This only works if you weigh it. You can't do this by volume.

You can always compare to known sugar; a comparison to know stevia is less useful unless it's identical, and brands evolve. The conclusion holds for fructose too (that's even more soluble), but glucose, which you're unlikely to have as pure powder at home, needs slightly more than its own weight of water to dissolve it.

Pure stevia is much sweeter than sugar, and when concentrated may have an aftertaste. It's also sold blended. When blended with other sweeteners the solubility test probably still applies (certainly in the case of erythritol, find in Truvia). Sometimes stevia is bulked out to make it a more direct substitute for sugar. The bulking agents may well be rather soluble and a source of carbohydrates (maltodextrin is sometimes used).

Ants don't care about artificial sweeteners.

Make two piles: one of your mystery sweet stuff and one of real sugar (as a control to make sure there are ants around). Maybe moisten them some or make syrup. Leave them outside somewhere you see ants. Then check them later.

Carbohydrates (sugar) are fuel for all animals. Stevia has no food value. Ants taste things differently than we do. They only care about stuff with food value: real sugar. They will not go to artificial sweeteners.

Experiment on yourself.

Or you could use yourself as a guinea pig. Eat the ketogenic diet with your wife. When your ketostix are good and purple you are in ketosis. Then eat a hog load of the mystery powder. Sugar will break ketosis within 15 min and your ketostix will go back to white. If it is sweet and does not break ketosis then it is fine for the ketogenic diet. That is what she really wants to know, anyway.

This answer is specific to OP's situation, and doesn't apply for the general case where "getting it wrong" has fewer consequences.

is there any surefire way to tell if this is Stevia and not sugar?

No, there is no surefire way you can tell.

Within the bounds of an everyday kitchen, and lay-person knowledge; there is no method by which you will be 100% certain of the contents of the jar. As this is for the health of your partner, who has a medical condition - it is not worth taking any risk whatsoever.

Furthermore; testing that the majority of the jar is Stevia, does not ensure that the jar only contains Stevia.

The cost of a new jar of Stevia, is far lower than the cost of accidentally harming your partner. I would strongly recommend not to take any risk, and accept the cost of replacement as part of your transition to a new, better labelled life-style.

Of course, the existing jar does not have to go to waste, I'm sure you have plenty of neighbours and friends who will happily make use of a jar of "probably Stevia".

Wow, do not throw it away, especially if you have a scale or yeast handy. There are 2 easy methods to determine whether the unknown sweetener is sugar or not.

Try fermenting unknown sweetener with yeast

With the exception of lactose, yeast can feed on all "real" sugars, or at least the ones you'd normally keep in your kitchen. On the other hand, in all of my testing, I was unable to get any "artificial" sweeteners to be fermented by yeast. Those tests included Splenda (maltodextrin + Sucralose), erythritol, and even allulose (psicose). The same would likely be true for Stevia granular sweetener, as it's a mix of maltodextrin + stevia.

Use weight and taste to determine sweetener type

First of all, pure stevia powder that isn't cut with anything would be incredibly sweet (with possibly a bitter aftertaste): the sweetness is much more concentrated than sugar. Generally, pure sugar substitutes like stevia extract, sucralose, etc. are usually "cut" or bulked with less sweet ingredients like maltodextrin (hardly sweet at all) or erythritol (75% as sweet as sugar).

Anyway,

1 cup of sugar weighs around 200 g.

1 cup of Truvia (erythritol and stevia extract) should weigh around 224 g.*

1 cup of Swerve (erythritol, oligosaccharides and natural flavors) should weigh around 204 g.*

1 cup of Splenda (maltodextrin + sucralose) should weigh around 27 g (yes, it's that light).

1 cup of maltodextrin + stevia extract should also weigh around 27 g.

If she's ever used this sweetener for baking, then it's unlikely that it's Equal (aspartame), as that loses its sweetness when subjected to heat.

* Note that any sweetener that contains erythritol will be easily distinguished by the extreme cooling sensation it has on the tongue compared to other sweeteners. With Swerve in particular, I've found the that the sweetness sensation seems to linger for minutes after I've tasted it (provided I don't eat anything else). I don't get the same experience with Splenda. Sorry, I don't know how this compares with sugar (sucrose), as it's probably been 7 years or so since I've tasted it.

OK, this one will not be very practical, but I was inspired by one of the answers who claimed that

there is no surefire way to tell

But there is.

Sugar is optically active. A linearly polarized light beam will rotate while traveling through sugar. There is an instrument that measures this called polarimeter and, actually, its main use in business is to measure sugar quality by this exact property.

So, if you somehow manage to get access to one, just check which of your substances does make the beam turn.

Put a pinch of each into a pan and heat it.

Real sugar will turn into brown and sticky caramel. Sweetener won't.