How do you determine whether a chocolate is sweet or semisweet?

Sweet, semisweet, etc. are American terms for the different chocolate "grades" and are determined by the amount of cocoa mass in the chocolate. In many other countries, terms referring to "darkness" are used.

But don't go ahead and use just any chocolate labelled "dark", because there are huge differences between brands. I have seen 40% chocolate marketed as "very dark" or "bitter" chocolate. You should look at the ingredients instead. They list the amount of cocoa mass (= the sum of cocoa solids + cocoa butter).

White chocolate is always clearly labelled as such, and seldom has a percentage listed for the cocoa mass. This is OK, because you can easily distinguish it.

Milk chocolate has around 30% cocoa mass, and contains milk in addition to the other ingredients.

Semisweet chocolate has at least 35% cocoa, but can be anything between 40% and 65% depending on availability and taste. Here you get into "real chocolate" territory. The only ingredients should be cocoa (can be listed as cocoa butter, cocoa solids, cocoa mass, cocoa liquor, or cocoa particles), sugar, and maybe some vanilla. Different forms of sugar are OK, for example organic chocolates often use raw cane sugar. Stay away from anything with vegetable fat, emulgators, and/or E-numbers listed on the package. If you have the choice, prefer chocolates with higher cocoa butter to cocoa solids ratio (but this information is seldom available).

Bittersweet chocolate is somewhere in the 70% to 99% range. It has the same ingredients as semisweet, just in different proportions.

Unsweetened or baking chocolate is essentially 100% cocoa mass, and may or may not be processed or conched to the same smooth texture as chocolates intended for eating or candy making, depending on the manufacturer.

Be aware that the limits are somewhat arbitrary - some authors count a 72% chocolate to the semisweets, not to the bitters. But I have found these ranges to work well. Usually, you have some leeway, because the texture of whatever you bake (and even of conditor chocolate items) doesn't change all that much with ten percent difference in the cocoa amount, there are factors which bring in a larger error anyway.

To warn you, almost none of the popular chocolate brands for eating are good for serious baking, including the ones you linked. Cadbury, Milka, Hershey's and so on are no good. Also, what you see in the baking aisle sold under the name "baking chocolate" or "block chocolate" is probably no good either. At least here in Germany, it is made mostly out of hardened vegetable fat mixed with some cocoa solids. Pure chocolate is too expensive to gain popularity. The cheapest one around here is Lindt excelsior (not the Lindor line which has vegetable fat) for around 2 Euros per 100 g. If you are in the supermarket, you should try looking through the ingredient lists of the "premium" brands to see if one of them is good enough. Else, you can try to find good chocolate online. Some stores will even sell true conditor chocolate (Varhlona, Callebaut) in household amounts, but this is still very rare.