look before you leap
Think of the consequences before you act, as in You'd better check out all the costs before you buy a cellular phone
look before you leap. This expression alludes to Aesop's fable about the fox who is unable to climb out of a well and persuades a goat to jump in. The fox then climbs on the goat's horns to get out, while the goat remains trapped. [c. 1350]