Looking for a phrase or idiom to say that only guilty people act guilty

I'm wondering if there is an idiom, phrase, expression or other figure of speech that would replace the following:

"People who are innocent, don't ever act guilty (therefore you must be hiding something)" Or "Guilty people are more likely to get nervous"

For example, this might be said by a policeman when searching somebody and they are acting suspicious. Another example might be if your significant other is acting suspiciously and think they might be cheating. The basic message is that somebody is getting bad signals from someone else and they will try to find out what they are hiding. Not sure if this is gonna make much sense to anybody. I'm really not sure if something like this exists but let me know if you have any ideas!

Solution 1:

where there's smoke there's fire

Merriam-Webster describes this as:

used to say that if people are saying that someone has done something wrong there is usually a good reason for what they are saying

Solution 2:

"If it walks like a duck, it quacks like a duck, it's probably a duck."

Duck test