First I read the rewards scheme to make sure it suits me. ThenI will do research about the ICO - product, team, whitepaper. Lastly I will check how responsive the bounty manager is
A lots of factors involved in it. I think researching project, reading whitepaper, team's background, social media activity, communication with the community and bounty manager's past works are important things for choosing bounty. If you choose the right project, you could earn great; but if you fail to choose right project, that bounty is waste of time for you.