The UTM grid is referenced by the numbers in blue in the extreme margins of the map.
An example, which appears on the 1:25000 map Number 3630 OT of the Massif du Mont Blanc, is that the Hotel de la Forclas (shown as Gite d'Etape) is at 32T 0345375 5102423
The 345 will be found along the top of the map and 5102 would be on the right edge of the map, were it not for an extra label occupying that place (try left edge!).
Click here to learn about the rest of the reference
[The image below is part of the relevant map, although the comments above refer to a whole map]