Practice.
I'll go through each character and fiddle about for 15-20m each. Go through moves/strings/specials, try to find and practice for a few minutes any obvious combo routes and spend a lot of time just dashing and jumping and moving around (movement has a lot to do with how well I like a character) then add them to a little list that's broken down in to Definitely - Maybe - No, in whatever category they seem to belong in. Then I go back over the Maybes and try to eliminate as many as I can , transferring the ones I don't eliminate to the Definitely column.. then I go back to that column and spend about an hour or so on each character, I'll hit up YouTube to see if anyone had posted any interesting stuff for them yet and I'll try it out if they have, but basically just spend more time on everyone I've got in the definitely column until I've narrowed it down as far as I can. While doing this I'll bounce out of practice mode and in to ladder or cpu fights to test and see how they work vs a moving target.. etc.
I spend, I dunno, 3-5 days doing this. Then I keep playing whoever I like well.enough and questioning my decisions on my main(s) for a couple weeks.
Then when the first tier list drops Ill immediately change to whoever appears to be broken and complain about everyone that's not my own character.