you should bring alone your camera with you to try. you should check there point:
1)are there any 花
2)does it look like it has been used before
3)check if the af isn't working
4)check are there any 移焦
5)make sure there are no missing things
P.S.:because I'm using nikon so that I dont know where can you find 2hand lens, but you should also check 1+3+4(+5). Hope you can find a great lens soon.