I don't know what they are doing now, but two years ago it was a three part, one day affair. They looked through our log books, gave us a really simple AIM/FAR type test (stuff like light gun signals, 121 duty time limits, etc), we took a personality/IQ test, did a one-on-one interview with a line Capt., then flew the ATR sim. The sim was very basic (take off from Albany, GA, hold at the VOR, some climbs, turns, etc, then an ILS approach (raw data) and landing).
I used the interview info line at AIRINC, it was pretty close to what I was asked in the interview. Bring some food, they don't give you a chance to eat.
Good luck!