How to find out if the coconut oil that I will buy from a supermarket is food and/or allergy safe
Since the question is off-topic, I'm not sure if I should be answering this. If not, I'll rely on those more experienced to edit or remove it.

If you have no known allergies to other nuts or seeds, coconut oil sold in the FOOD section of a supermarket is safe. If you have allergies to any nuts or seeds, then you'd need to check for labeling saying if it was processed and packed in a nut/seed-free facility to be safe.

On the other hand, if you're wondering if you might be allergic to coconut itself and you don't know, think back to other foods you're eaten. Coconut is in many foods and I'd be surprised if you haven't eaten it before. It's found in a host of different candies (even as small bits of crunchy toasted coconut), puddings, cookies, cakes and other baked goods. It's included in different curries; sometimes finely ground like flour or as coconut milk. I'm sure there are other foods but this gives you an idea.

If you simply don't know, you might try a very small amount of dried shredded coconut and wait 24 hours. Initially, allergic reactions are generally mild as a full allergic response normally takes 2, 3 or more exposures to cause a full allergic reaction. Perhaps keep something like Benadryl on hand. Mind you, this is what I'd do but please use caution. You may not feel like trying this.