Remove the prop, look at the back side for signs of rubber. you won't see much just black in the groove between the prop and its hub. It is more common for the prop's hub to slip than the coupler, usually you can smell the rubber burning when a coupler fries.
Phil
Added note note I just read your post "Scots Donzi", Can you be a little more specific? There is a difference between slipping and jumping in and out of gear. Like fix said it also could be the clutch dog failing, you may hear the ratcheting or also feel it come into gear kind of hard. Hub slip is more of a soft engagement.
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.