Unfortunately, the way I have determined a cat to be clogged in the past was to partially unbolt the front connection flange, then test drive to see if the problem went away. A little noisy, and not very 'technical', but definitive!
If your 124 is setup like mine, it is all one piece, so this cannot be done.
Seems to me I saw a post go by on how you can test for this with a vacuum gauge. You might try a search.
1986 300E 5-Speed 240k mi.