How do you accuse your partner of cheating?

You can't do it manually, you have to catch the partner and the new partner being romantic, and your Sim will automatically accuse the Sim of cheating and get angry.

my sim literally walked in on them making out and she didn't even care? like she just stood there......

@Herbert_the_Boo  I think that the partner has to witness the start of the interaction, or it doesn't always register.  If you'd like the couple to fight, the cheating sim can "confess to cheating."  There's also a chance for an autonomous accusation every time they see each other after being apart for a bit (like while one is at work).