Several things to look for!
First, you have to check the rear of the receiver to ensure it is straight cut, and not a slanted receiver. Straight cut allows standard type-56 furniture to fit, the slanted receivers have a funny angle and require special furniture. If the original thumbhole stock is present you may need to remove it to check.
There are also some, rare-ish underfolder receiver MAK90's and if you live free from an assault weapons ban, reinstalling an underfolder isn't that hard.
Next check that the fsb is straight and true by looking down the sights. If the frontsight post looks crooked, or is severely drifted left or right for windage, it is probably crooked.
Check that all of the major parts match.. Receiver/action, bolt carrier, bolt, gas tube, action spring guide/top cover retainer are all numbered, top cover may or may not have numbers.. mine does and matches.
Edited for spelling and a couple sentences..