What are the important marks to look for on a yugo sks?

You're really not going to find anything which is going to be significant to the value of the rifle on the M59 or M59/66. Those that saw service during the Yugoslav wars of the 90s may have some markings indicating which side used it, whether through metal pressing or hand carving (the latter is typically on the buttstock), but the only thing that'll really increase the value significantly is if you have an M59 in decent condition (the M59 was more closely patterned after the Russian SKS, and doesn't have the grenade launcher or gas cutoff valve), or an M59/66 in unissued condition.