You correctly identified the lane in which you should NOT gank, at least by yourself. Ganking a losing lane, unless they are very low and/or you yourself as the jungler are equally or more fed than the enemy laner, is in most cases the most incorrect thing you can do. If they just got a solo kill on your laner, and dont have any items yet, yeah go ahead and shut his ass down. If they are 5 kills on you and your laner is getting crushed...no bueno, let it die.
If you have ever watched a pro stream while jungling, you will hear them occasionaly say something like, "That is a dead lane" Meaning: the lane is lost and ganking will only worsen the situation. The guy yelling at you is worse at league than you are, ignore him.
The correct thing to do when a lane has lost that hard is to:
1,) Let it die.
2.) Equalize the pressure in another lane and gank the snot out of them until the snowball out of control
3.) If you must gank the lane to protect inhibs or towers, BRING YOUR TEAM!! At least one other team member, maybe the support or other solo laner should come with you to ensure the guy gets shut town. Because now that the fed guy is down, and you didnt die you can relieve some turret pressure, get global gold, and get some map control back.
Now on to when to gank.
1.) enemy laner is low or oom
2.) your laners are healthy - they have mana - and the enemy is pushed up reasonably close to your tower so they have a long way to safety
3.) your ults are all up and you can chain cc ult them into sad panda land
hope i was helpful.
