carkrazy01

Do you have any instructions for the plane? If it is a high wing or mid wing plane, you will balance it upright. If it is a low wing, turn it upside down. Assuming you have the instructions, you can measure back from the leading edge (or wherever the instruction say) and place ink marks there. It's easier to mark it if you put masking tape on the wing and put your marks on that.
Slightly lift the plane at the marks on your fingertips, and see which end drops. Be sure you put your marks over the part of the wing near the fuse where there is some balsa sheeting or your fingers may go through the covering.

If you don't have an instruction book, balancing the plane on the middle to the back edge of the main spar will get you pretty close.

Better yet, tell us on here what kind of plane it is; somebody probably has the information.

Good Luck
Larry