Well … you can safely assume that MOST tracks are in key as you get them. If you change speed of the tracks to make them work in a mashup, be sure to have keylock on. That way the key doesn’t change when you change the speed.
Try keeping your speed changes limited to 3-4 bpm either way.
Usually if something is off key you can tell. For the musically trained ear it’s possible to tell where it’s off, for less experienced ears there’s that “feeling” of something not being quite right.
Hope that helps and some of our musically inclined readers have more answers for you.
Greetinx.