How do I know if my cast iron dutch oven is preseasoned?

You would know if the dutch oven was covered in wax. Cast iron prior to seasoning is a light grey color. It will rust very quickly just from atmospheric moisture, so they cover it in paraffin wax to preserve it. The process of seasoning the utensil forms a tough, yet smooth, black coating of polymerized oil. If your dutch oven is a slightly shiny dark black, it is safe to say that it has already been seasoned. With the proper care your dutch oven will last you many lifetimes-- enjoy it!

If you bought it new anytime after about 1930 then you can be sure it's pre-seasoned. Every "modern" manufacturer pre-seasons their cast iron cookery.

Though wax was most often used as a protective coating for unseasoned cast iron in the era when purchasing an unseasoned cast iron pan was an option, it is still often used to this day by manufacturers who pre-season their product, just to protect it from filth accumulation during shipment and storage. For that reason, a wax coating can't reliably suggest seasoning or not.

The real test will be considering its color. Unseasoned ("raw") cast iron looks exactly like you might think untreated iron SHOULD look: a shiney silver grey. More accurately, it would remain shiney silver gray for about 2 hours before being almost completely covered by a thin film of red rust it would begin to accumulate almost immediately after being exposed to the air (I've stripped down many pieces of cast iron cookery and it's been my experience that visible rusting develops on raw cast iron within minutes of exposure to air. This is, in fact, precisely what the seasoning is intended to retard). Probably every piece of cast iron cookery you've ever seen is dark black (forgetting the common exception of ceramic coatings, which are often colored blue, red, yellow, white, etc.) That dark black IS the seasoning which covers and protects the raw shiney silver iron underneath.

Short answer: If it's black, it's seasoned. Even shorter answer: Everything is pre-seasoned.

You should check the manufacturer's instructions on what you need to do with the pan.

Odds are that if it didn't say 'preseasoned', it isn't ... and so the manufacturer should make some recommendations on how to strip whatever their protective coating is, and set up your initial seasoning.

For the stripping, some will tell you to scrub in hot water; others involve potatoes or a salt scrub. If this is a new purchase, and there weren't instructions with the dutch oven, that you check the manufacturer's website ... if they don't have something there, contact them.