The Prins reaction – stepwise addition of an alkene to a carbonyl compound – leads to the same product as a concerted ene reaction. This is illustrated here with tin tetrachloride as a Lewis acid, which (as far as I know) lowers the activation energy of both pathways:
How can these two mechanistic pathways be differentiated experimentally?
One thing that I can think of right now would be the stereochemistry of the alkene formed - presumably in a concerted pathway, the cyclic transition state (envelope-like, if I remember correctly?) the substituent R' would be positioned equatorial or pseudo-equatorial. However, I'm not confident enough to undertake an analysis of the stepwise reaction.
I'm open to hearing about other possibilities, e.g. kinetic studies or spectroscopic studies. I suppose the free carbocation in the Prins reaction could lead to a more negative coefficient $\rho$ in a Hammett analysis, compared with a stepwise mechanism?
I'd also be glad if somebody could provide an example in the literature, if it exists. (I confess to not having looked this up - been very busy.)