Before every potential gank, run through this checklist:
1. Is the enemy pushed past the river? If no, don't gank. You can't gank someone under their tower.
2. Does your laner have CC or burst to follow up? If no, don't gank. You can't solo-kill a full-HP enemy at even levels.
3. Is the enemy jungler on this side of the map? If yes, be careful — it might be a 2v2. Are you stronger?
4. Are your camps up? If yes, consider farming instead. Camps are guaranteed gold. Ganks are not.
5. Is there an objective spawning soon? If yes, ganking the lane near the objective creates a numbers advantage for the fight.
If you answered yes to at least 3 of these, gank. If not, farm.