I feel almost like a traiter writing this as I was that guy once but when I was bad even though I carried on using I deep down was desperately hoping to be caught out so I could get it out in the open and address it and deal with it. Which eventually happend and my recovery started.
What I'd say is if you're home together the most. Obvious thing will be the person's behaviour. Cocaine without alcohol makes you very edgy, very jumpy almost nervous. People have the idea that it makes you very cocky very bouncy like in the films. It often is the oppersite when used without drink. Eye contact is important too, when on it eye contact will be avoided. You'll notice them like scanning the room constantly but not looking at you.
Then theres also things like being sweaty, not being able to keep their hands still. Runny nose etc
Are they going to the toilet alot more and shutting the door? Men don't normally shut the door if they are going for a wee.
Then there's the question of where it's being kept. Probably on his person. So check wallets pockets etc.
The floor is a bug one people miss. When you do a line you wipe down the surface but often specs of it fall out your nose and will land on the floor or surfaces near by. Also if a surface has been wiped down and you go in straight away after, question why the side has been wiped down and is wet.
Finally it sounds silly but crouch down so the surface is at eye level and shine a touch when it's dark you'll see then any specs that have been missed.
If it is the case your partner has been using, I know you'll be mad but there is normally a deep rotted reason for use that is more than just wanting to do it for fun or what ever. Your partner is probably struggling with something and please remember that when confronting them.
All the best and hope that helps