Trust in kosher

I let my house to someone who is not religious for a week. We agreed in advance that he would not use my oven, pots, and utensils, for kosher reasons, and he said he had no problem with it, he never cooks anything.
I came back and I have a strange feeling that he did something, I don't know, maybe he used it in an unkosher way, it's something in the speech and the things and the mockery of the kashrut part. I can't say exactly what aroused my suspicion, but I really suspect it. He says that as we agreed, he didn't use anything except for one-time use. The guy left and that was it.
Do I need to legalize everything? This is a rant, but if from a halachic perspective it is "appropriate" to legalize then I have no problem doing what is necessary.