If there is a fully automatic TLR then I would be surprised, however some models have built-in light-meters.
As we have not been told the specific camera, presumably the film-speed dial is either on an in-built meter, or on a film-speed reminder. In the latter case there is no problem of course. In case the meter-dial is stuck, just open the aperture one stop or go to the next slowest shutter-speed in order to double the amount of light in your exposure.
edit: Hahahah, looks like we are all suggesting the same . . .