Get a large styrofoam cup and fill it with crushed ice. Add cold water and allow to sit for 5 minutes. Loosen the lock nut on the back of the dial and submerge the shaft in the ice water. Allow to sit for 60 seconds, then twist the dial face so the needle alligns with 32 degrees F/ 0 degrees C. Tight the lock nut. You thermometer is now calibrated.
If you have access to lab thermometers (mercury-filled) you can perform the procedure at different temperatures- preferably at your preferred developing temperature.
Watch out for the electronic digital thermometers- my experience with the three or four I've tried shows that they are not that accuarate (plus or minus 3 degrees to a calibrated lab thermometer) and worse, they tend to 'wander', giving slightly different readings of the same temperature fluid over mutiple measurements.
Those Weston's are the bomb. I have one thats about the same age and has carried me from the self-made "closet" darkroom to the halls of MoMA- and it still waits for me by the darkroom door with its leash in its mouth when I show up with a bag of plates.