sometimes, someone might try to prank you and switch or flip a few pieces. this is especially bad for beginner solvers, so i hope this post can help anyone who needs it ^_~
so, anyways, how to tell: THIS IS FOR DAN BROWN METHOD SOLVERS
1. just solve two layers
2. if there is only 1 or 3 flipped edges for the top cross then it's impossible
3. continue solving
4. if corners are in a 2-cycle or 4-cycle it's impossible
ok, so example, 4 corners going clockwise
a 2-cycle is when only 2 pieces are switched with each other, like
c d you see c and d are switched
a 4-cycle is when all corners are out of place, and they need to travel in some sort of orbit
a c all pieces
c d is NOT impossible to solve
lastly, for the corners...
check their orientation,
if it's orientated call it "0"
if it needs to go counterclockwise call it "4"
if clockwise, call it "2"
then add it up, if it's not 0, 6 or 12 it's impossible
so, if it's impossible, just take the pieces out and put them back in properly, and say to whoever scrambled it, "Dude, you took a piece out right, because i can tell. you failed."
if you have any questions just ask