Heading over to morzine/les gets this summer and have been looking at IGN maps to try and find some other singletracks away from the main bike parks.
http://www.walkingclub.org.uk/ign_france_maps/france.shtml

I can’t seem to find a key anywhere, but is it possible to tell what trails you are allowed to cycle on?

Many of the trails have a purple solid line, some with black dots, or black dashed lines.

Purple solid lines denote a GR (grande randonnée) which will be waymarked in red and white, the long dashed purple ones are local randonnées, usually waymarked in yellow. The shorter dashed purple often aren’t waymarked.
The fine black dots and dashes show where the commune limits are, they may follow the line of a path or track, but they aren’t a physical feature.
Hth

My experience of riding paths in France is that unless there are local restrictions (and the only one that I know about is Chamonix) is that the vast majority of paths are good to go.

Combine the IGN map with Google Earth, the strava heatmap and Strava segments for a better idea* of if the trail is rideable.

*Might be poo.

You could try this website for some ideas….

http://www.singletrack.fr/sentiers/sentiers.php

Purple tracks are waymarked walking trails (GR or PR)
There’s no national restriction on cycling, only local byelaws.

However, many randonee trails are rubbish VTT trails.

Cross reference other sources with IGN to mark up your own routes.
Use VTT topo guides
local VTT websites
https://www.utagawavtt.com/
and
Strava heatmap.

also more info here:
http://singletrackworld.com/forum/topic/help-please-morzine-for-the-non-downhiller

Why don’t you just look at the Ordnance Survey map for it?

Seeing as the UK has annexed it along with Chamonix there must be an OS available now?

Op, the easier question would have been
“Where am I not allowed to ride my bike in France?” 🙂

Outside of some cases, such as Chamonix, acess is restricted in the national parks (each one has it’s lists of permitted /forbidden trails), but there aren’t any next to Morzine. For all the rest of it, DBAD.

Excellent cheers 😀