how to identify lumber grade of unmarked ceiling joists
Code requires a Live Load of 40 lbs. per square foot plus a Dead Load (flooring, joists, insulation, etc.) of 15 psf (probably) for a total load of 55 psf. 

#3 spruce/ fir (fb = 1200 psi &amp; fv = 95 psi) spanning 14’ will support about 40 psf at 16” oc.      #2 (fb = 1750 psi &amp; fv = 120 psi) will support about 60 psf. The main difference between #2 and #3 are knotholes. If you have loose or missing knotholes, then it’s probably #3.  If the knotholes are less than 2” in diameter and “tight”, then it’s probably #2 and better. 

Code requires 1-hr. fire protection between a garage and “habitable space”.  Therefore, you’ll want 1/2” Type C or 5/8” Type X gypsum board on the bottom of the floor joists, wall by stairs, etc. for protection.

The door leading to the upstairs needs to be rated: 1) solid core, and 2) frame needs to be solid material (no applied stop), 3) weatherstripping around on 3 sides... but consider threshold too due to fumes. 

If you use L/180 the gypsum board will crack under deflection. I’d use L/360...then, no cracks.