In my case my ex was 'going to a weekend business conference' ....I found a piece of paper with the name of a hotel in BRIGHTON of all places ! (he said he was going to Norwich) so I rang the hotel and asked if he was there with his ;wife; and was told yes, but they had gone out ,,,,,so I said well, this is his wife speaking, so could you please leave a large note on reception asking him to ring his wife as soon as he gets back. He didn't, but the he arrived home that same night ....all the usual excuses, etc ....so I told him he could take his washing to her,,,got the locks changed and that was almost the end of that !!! An idea.....say nothing , but prepare ! go and buy yourself that same bra, wrap it up in some pretty Christmas paper with a blank label ,,,,and on Christmas day you can open it in front of him and exclaim ;Ah how thoughtful ....a pretty bra, just the one I wanted ...how did you know ? and then watch his face ......
he will either be horrifed or surprised or pleased ...the first if he has been found out ...the second wondering who gave you that or the third ..hoping for an excuse to tell you about his cross dressing !
Bon courage !