How can you tell if a coin has been in a fire?

well it will be black if not already melted.depends on how hot the fire is and the type of metal i guess eg if its aluminium it wont melt it my become black and have some wear.

Also according to the heat it may have just turned color. Hard to describe the colors though. Copper will normally turn a pinkish color.

I've got some Kennedy halves that were in a fire. Bubbles on the surface and permanent black marks.

Besides the obvious telltale signs, if the coin is a clad/composite coin, the metals will have begun separating. I've noticed this many times with all US modern coins (cept for the nickel) when I put one in a flame).