What you do mean with join? There are different sorts of joining up either as an investor or a marketing/bounty participation.

If you do mean up as an investor then decision would most vary or depend on your research. Knowing projects feature, team or developer behind it, partnerships and all other related factors should really check of.
If you do mean as a bounty participant then always go follow into those projects handled by famous managers same as the guy above me mentioned about that matter,most likely project being handled by them do really succeed.