.
Like all professional railwaymen, SNCF on board staff will generally invoke the rules - always politely in my experience. If a service is reservation obligatory you should obtain a reservation otherwise you could be fined. However as has already been said, only a few key non-TGV routes 'Intercites' are reservation obligatory.
That said:
The situation has been muddied since SNCF Intercites sector abolished the TEOZ brand which had been created a few years ago to be the 'quasi TGV' top link trains on the lines to France's main cities that were not up to that point served by TGV. At the time this equated to Paris - Strasbourg, Paris - Clermont Ferrand and Paris - Toulouse
Best way is to look at the timetable leaflets which are listed under each route. The list of routes is on this SNCF Intercites page:
http://www.intercites.sncf.com/reseau-intercites/carte-reseau-intercites-lunea-teoz
The timetable fiches now seem to be the only way to discover which Intercites trains are reservation obligatory
eg Paris - Limoges fiche horaire where you can see the main intercites services are marked with the reservation obligatory symbol
http://www.intercites.sncf.com/resources/LigneFiche/315/FichePdf.pdf
compare with the trains to Bourges/Montlucon where there is no symbol on the timetable:
http://www.intercites.sncf.com/resources/LigneFiche/311/FichePdf.pdf
or Paris - Cherbourg which is non reservation:
http://www.intercites.sncf.com/resources/LigneFiche/309/FichePdf.pdf
ps
The wikipedia blurb gives a decent explanation of what Teoz was, and is correct about them being irritating as reservation obligatory like TGV but without the speed of TGV. However it fails to note that former Teoz services are still reservation-obligatory
.