Is there any way to tell if dark skin under my big toenail is a bruise or not?

Dark mark: Hello. If you recall an injury, then it could be a small hematoma or bruise. It can last several weeks and like a bruise on the skin should change in color, size and even maybe in exact location. Could also be nail fungus (make sure it is not on the toenail). Lastly, if it has been there a while (a month or more), i would recommend a dermatologist see it. Rarely melanomas can be under the nail.

The tissue: In question can be sampled. If at the edge of the nail, it can be clipped and looked at with the naked eye. A history of trauma would also correlate with a bruise.

Poss. benign tumor: Another possibility is that this is a benign tumor called a glomus tumor. Get it checked out by a dermatologist or orthopaedic foot and ankle surgeon.