Most samurai armors on the market are assembled with elements from other armors. On older Japanese armor, the association of different parts can be part of the samurai armor's history, while for those made during the Edo period, it would always be better to check that all the pieces are consistent with each other. Here are the five most easily verifiable points.
Lacing color The most obvious element of samurai armor is the lacing (odoshi) that joins the plates of the various elements. Making sure the colors are the same, and not just similar, is definitely the first step to take. Be careful though that some elements may be intentionally tied with a different color from the rest of the armor.
Coherence of linings Some elements of the original Japanese armor have a silk brocade lining. These are generally the thigh guard (haidate), the arms (kote) and the shin guards (suneate), to which we refer in the complex as "sangu", or "the three elements". It is easily verifiable that the brocade present on these three elements of the samurai armor is the same.
Consistency of the "kikko" Suneate are often provided with an armed hexagonal quilt, called kikko, as it resembles a tortoise shell, protecting the knee. The same padding can be found attached to the armor of the armor (dō): if they are of a different color, the suneate did not belong to the armor!
Consistency in style Look carefully at the last plate of each element. They should all have, or almost all, the same type of decoration and finish. If this is not the case, become suspicious and thorough.
Coherence in protections Kote and suneate are almost always constructed in a similar manner, with metal guards of the same type. If they are very different, you should proceed with a deeper analysis of the individual elements.