How to tell exactly what part of the joint wont close? (M&T/groove on breadboard end)

I’ve got a M&T/grooved breadboard end (5 tenons all with a haunch or groove between them). It’s a bit snug up to the last 1/4” or so then it wont go anymore…

Except, it feels like I can get either of the two ends almost closed – meaning that the problem is probably in either the groove or the 3 center tenons… Having accidentally taken too much off of tenons trying to fix this on another project, does anyone have tips on figuring out exactly which part of the joint is the offending section?

I don’t want to uniformly shave down my tenons and end up loosening one of the M&T’s unnecessarily.

The offending (yet attractive) breadboard end: