Idiom similar to “if the shoe fits”

Sounds more or less like A guilty conscience needs no accuser.

A feeling of guilt and remorse can be so strong that it will prompt an offender to confess, even if no one is requiring them to do so.

Gary felt so guilty after taking the money out of Bill's wallet that he confessed and returned it a day later. A guilty conscience needs no accuser.

[The Free Dictionary]

The expression '[that must have] hit a raw nerve' is often used about an unexpected reaction to a remark (or sometimes an article):

hit a (raw) nerve

To evoke a strong emotional reaction, such as anger, sadness, or disgust, upon being encountered, heard, read, etc.

• Your column must have hit a nerve, because we are getting slammed with feedback from readers – and they're not happy.
• I could tell he was hitting a raw nerve when he brought up Jane's former employer and Jane went silent for a moment.

[Farlex Dictionary of Idioms]

The context 'it must have' / 'you could tell she'd hit' etc forces the 'non-contrived' knee-jerk reaction.

A common (and arguably idiomatic) allusive expression is methinks he [or she or they] doth protest too much. The source of this expression is Shakespeare's The Tragedy of Hamlet, Prince of Denmark, where it appears in a somewhat different form, during a critical scene in which Hamlet has arranged for the (new) king and the queen to watch a play that reenacts what Hamlet suspects actually happened in the events leading up to the murder of his father, the previous and rightful king of Denmark:

Hamlet[ to Queen Gertrude]. Madam, how like you this play?

Queen. The Lady doth protest too much methinks.

At this point in the play within the play, the "Lady"—who is acting out the same role that Gertrude played in real life—has been effusively professing her extreme devotion to her husband, who is soon to be murdered by his brother (who then usurps the the throne), with the queen's connivance.

The sense of the expression is that people characterized as "protesting too much" are overdoing their show of innocence or virtue and thereby inadvertently hinting at their dishonesty or lack of virtue with regard to the matter under discussion.

I think you may also be interested in the phrase too close to home.

I use it/see it used when something is unintentionally very close to my specific experiences. In your 'NOT ALL MEN' scenario, someone who wants to point out that the male speaker is getting overly defensive might rhetorically ask them "A bit too close to home?" implying that the male speaker shouldn't have grounds to be defensive over other men being jerks unless they have inherent misgivings about their own actions.

Hope that helps.

Sounds like: The guilty defend when no one accuses.

I think I need to note that this is something the various “how to identify liars” seminars/training programs teach, and as with everything else they do, is total bunk, and in real life ends up with innocent people behind bars because the cops were certain the person was guilty. If you’re using it for a story or something like that, it’s fine, if you are using it where there are real consequences for getting it wrong, don’t.