To cash a check that has someone else's name on it, you'll need to have the person sign the check over to you by having the person sign the back of the check with their name and pay to the order of "Your Name Here"
Verify that the bank will cash the check though as some will and some won't.
All the person has to do though is write pay to the order of on the back of the check in the endorsement area and then they write your name below that.
Then you endorse the back of the check as well.