Probably you will never be 100 % sure if given bounty campaign is legit and you can trust on it or not. The truth is, that you should do your own research. If you invest in given alt coin, before you invest, you read and check everything for the ico. In the bounty campaigns is the same. In fact, you indeed invest in a bounty campaign. You invest time.
Also, it is very important who is the bounty manager. There are some bounty managers, that are very popular and their campaigns and the ICO that promote are ok.