to know the bounty gives big rewards depends on proper research about the project itself, the medium one will use to participate in that campaign, the value of the coin, the nature of the market at the time it is paid, and luck. it is also opening one mind to the fact that some will pay and some won't and the ones paying good value are few. also, be on the outlook for scam projects.
I feel it's much better to first find projects you are legitimately interested in and once you find such projects, then you should find if they have a bounty campaign and then join them. If they don't have a bounty campaign you can always still find ways to earn some free tokens by joining their telegram group or following the on Titter and taking part in activities actively and even asking admins if you can do some promotion and get tokens in return.
I feel like that's a more standard approach and one we should all look after, rather than chasing big number of tokens which are useless after the presale or fair launch. I have seen some cases where the project owners don't even pay out the promised tokens.