right now if you want to invest in any of the altcoins i suggest waiting a while because it is not a good time in altcoin market and prices are falling fast.
but after this period:
What sites do you check?
mostly i check the charts on the exchange! but apart from that i check bitcointalk and coinmarketcap for additional info.
How do you guys select the ones with the potential?
What are your criteria?
Do you have some system?
I believe you have incentive to share this information because more of us invest in one coin, more the price is.

a coin that has enough volume everyday, and sometimes if the project has some interesting features that may sound interesting to me.