Generally what controls the CPU a board can take is the 1) socket and 2) chipset. So look at processors compatible with the board's socket and chipset. The service manual for the motherboard, usually easily available with a quick search, usually also tells you (on page 6 in this case).
- Support for Socket AM3 processors: AMD PhenomTM II X4 / X3 / X2 (except 920 / 940) / Athlon II X4 / X3 / X2 / Sempron processors
On that page of the manual:
- Six-Core CPU Ready
Also, ASRock appears to have a list here.
Also, read the answer to this question. The BIOS and motherboard also must support the CPU you want to put on the board.