There should be a number below the amount in a storage saying how much of that recourse can be raided.
Also they should add a reward system along with the league bonus, for every star you get, you get one gem!
Please add these they are cool!
Assuming you mean when viewing someone else's village, it is not difficult to estimate how much can be raided. Let's say you are a TH9 viewing a TH8 base with 253,621 in one of the storages.
In this case, that 253,621 storage you saw would net you right around 45,000 in a raid. The actual calculation for 253,621 (assuming equal amounts in all 3 storages) is 45,652, so we're close enough for government work.
If you think that takes too long to do, there's no time limit when you are viewing a base. When you are actually attacking, you can't get more than a vague idea of what's inside a storage anyway, so there's no need to do this type of calculation.
If you want to view the number on your own storages, that all depends on who's raiding you (due to the loot penalty). You could have up to 7 different answers.
Not that I think it's a bad idea necessarily, but in the meantime you don't have to be in the dark regarding what you stand to gain.
I definitely agree with the additional bonuses for more stars, though. Maybe more resources, though, rather than gems (although I wouldn't mind gems, I just don't see Supercell agreeing to that).
Actually once you do it a a few times you'd be surprised how quickly you can calculate it in your head. I'd say that takes no more than 4-5 seconds to do at most. Remember it's just an estimate, so round the numbers off to make it easier.
Plus, there are tools online which can calculate for you.
Niiiceee, i'd like to get gems in another way
What do you think?