Being a homebuilt, it could be done any number of ways. There is no plans for how to attach those stick grips.
My Dad has an RV-7 with those grips and a set screw, drilled and tapped, on the front side fairly low on the grip to be hidden out of sight. I recently installed some in my RV-4 and used hot glue as a temporary solution at first. My grips have started spinning slightly as others have said and I'll shortly be drilling and tapping for a set screw like Dad's.
If yours have no visible set screws, they're probably glued. If it doesn't come off easy (like my hot glue debacle...) then it might be Epoxy. In that case, best of luck to you getting those off. You might have to break the wood to get them off or just buy new stick tube. Tubing is fairly cheap compared to the damage you could do.
RV-4 N249P O-320, Dual P-Mags, Warnke Prop, GRT Sport EX