I mine just a few alt coins at a time typically -- but I manually switch to them when I'm ready.
I typically look at the current difficulty - where it is going - the current price and where it will go.
And then you need to look at other major coins (and what they are doing) -- as many people will switch to the "most profitable coin" at the moment -- which leaves a lot of opportunities with other coins that are being neglected
It is very difficult to predict the price and difficulty.
But there are trends and signs. If a coin all of a sudden gets a huge hash behind it -- for no real reason on the surface, then you know something is up and more investigating is needed.