I found a key in my wife bag which doesnt belong to our house?

i found a key in my wife bag which dosent belong to us? is she cheating on me?

That's one possibility but there are thousands of other reasons she could have a key also.

When your relationship gets to the point that you feel the need to search her purse, there is something seriously wrong going on between you. Instead of focusing your investigation on one inanimate object, try to understand what the real problem is and direct your efforts to correcting that.