I would focus on trying to make him feel like he can be honest with you without being afraid as opposed to trying to catch him out somehow. I would definitely not start snooping through his stuff.
Instead I would think about why he might be using drugs and what you can do to make him feel more comfortable talking to you about it. These are just my thoughts on the matter both as having been a drug addict in a relationship with someone who was not and having been in a relationship with a drug addict when I was not. I am now clean, by the way, and feel a huge part of that is due to the way my current partner has treated me without judgement and with kindness, support and welcoming honesty.