I'm looking for the market to tell me this.

The filter I use is: keep an eye on top 20 in market cap.
Then follow increasing 24h volume.
Once volume is steadily increasing for a number of days, get in.

This method got me into DASH when it was trading at $20... and on ETH when it was $40
I'm now entering XEM (or NEM) although I must say I don't much like the activity around it: it has little to no social mentions and even it's own forum isn't that much active.