anything with gills leaves one.
you place the cap/top section minus the stalk on a plate of glass and cover to keep moist for about 4 hours, sometimes overnight
lift off and see what you have left. usually its a coloured spore print. i.e. everything thats dropped out whilst its been sat.
oddly fun to do but you can look up the colour and pattern in ref books along with the full descriptions, i.e. location of growth, size, colour, cap features, gill length, gill characteristics (free, adnate etc)
this can help to identify although nothing is ever 100% accurate (sadly)
have fun!
pete