just think about what the other champion can and can't do and compare it to what rek sai can and can't do.
for example, in my head when about to gank as wukong
-does the champion(s) have a escape? no
-did they burn flash? no
-do i have ult? yes
-does my ally have a stun or a strong slow? no
go in w+q and walk away as they flash out of fear, if their low enough i'll ult knowing that the knock up damage will secure the kill.
have that same frame of mind when your going to gank lane with any jungler. just note that everytime you early gank, you may put yourself behind a level compared to everyone else. lol is like chess (the cheesiness is real), you anticipate what the other guy may do then set a trap.
also you should pay attention to their positioning, if their on your side of the lane and your about to gank, go for it, they are most likely they are going to use their flash though which is good in a sense. if they are in the middle of the map, let them push in a little further, or gank if your teammate has hard cc, if you don't they'll flash and escape or burn they're escape ability, if they are on their side of the map, don't bother unless they are low enough that you can kill them in one hit and they don't have flash.
also, one last thing, ping to let your teammate know your coming, because sometimes they might not be aware that your about to gank, it will really catch them off guard
