Look at his eyes when you ask him. If he looks to the left, he is using the creative side of his brain and is thinking up an answer. If he looks to the right, he is accessing his memory and is telling you the truth. If he looks straight into your eyes he is telling the truth, and if he looks down, it's usually a sign of guilt.
A condom doesn't necessarily mean cheating, some men us condoms when they're by themselves.
I agree, a spell isn't the right course of action here. You need to confront him in a non assuming way. Don't be aggressive. Merely ask him "why do I keep finding these?" Trust is a two way street, eight years is a long time, and you need to give him the benefit of the doubt. Otherwise, if you show him that you've lost trust in him, that will hurt the relationship and it will only cause an argument.
I've been married 7 years, so I'm not far behind you, and I know that the absolute worst thing to do is accuse your partner of doing anything and jumping to conclusions, even if its as simple as finding something broken. Being calm and inquiring over what you find in a non convicting way is best. In the early years of our relationship both my husband and I have put our foot in our mouths more than once and hurt each other in the process. We realized that assuming wasn't a good thing the hard way, but we learned from it and have a far more understanding and close relationship because of it. Our trust actually strengthened because we discuss things. And that demonstrates trust in itself to be able to speak openly and be listened to with an open mind.