The first question that you’ll want an answer to is one which is impossible to immediately answer in most cases: What color are the spores?
Since the spores are microscopic, you can’t just look at one and see what color it is. To properly find the spore color, you must get out a sheet of white paper (some people prefer half white-half black paper), cut the stem off the mushroom, put the cap on the paper with the gills down, then cover with a bowl or cup. If you’re lucky, after a couple hours or overnight you should have a nice outline of the gills made by thousands of spores falling onto the paper, like the one shown, and can easily tell the spore color.
The problem, of course, with this method is that it takes at least a couple hours and sometimes if the mushroom is too dry it doesn’t work at all. What most experienced mushroom people do is make an educated guess based on the color of the gills and their past experience. It helps that there are only a few major colors that spores usually can be: white, pink, brown, or black. (There are actually quite a few varieties of browns and off-whites.) One can start making a spore print while making a tentative guess as to what color the spores will be.