How to detect a Shapechanger?

[ShadowMaster]

According to the monster manual, an Aranea can take any "humanoid" form. How is it possible, for PCs to discover the thrue nature of such "monster"? Does detect magic reveal that the creature is magical? What about true seeing, does it reveal the thrue form of the spider?

Thanks.

 

[James McMurray]

Yes and Yes.