There should do some research before join bounties but is also a risk even if check before because no one can guarantee that the price of a coin will grow or will stay low after reach exchange.
For me it is really hard to find a solid bounties right now but i think the best way to identify a good bounty is to check if the project is good and if there telegram community is strong, and off course if the bounty has a trusted manager for sure the bounty is good.