The standard procedure for checking fluid in assemblies that have just drain and fill plugs and nothing resembling a dipstick is:
1. Raise the vehicle SAFELY as needed, but keep it level at the same time so as to not affect the fluid measurements
2. Check the owners manual or other documentation to see if there is a hot or cold way to measure; on differentials and manual transmissions as a general rule on passenger cars there is no hot or cold value specified
3. Clean the area around the fill plug thoroughy.
4. Remove the fill plug, being prepared for a small amount of warm-to-hot fluid to drain out; if fluid drains out, generally speaking it's full! :lol:
5. If no fluid drains out, stick your pinky in the hole and bend it down, trying to feel for the fluid level; be mindful of heat
6. If no fluid drains out, be prepared to add the correct fluid to bring the level up to the point where no more can be added without it draining out (or up to the point described in the specs); clean the threads of the plug hole first to prevent any debris from getting in
7. After adding fluid if necessary, clean the fill plug threads, use a little teflon tape if the plug was leaking, replace the plug and tighten to the right torque spec
8. Lower vehicle, road test, check for leaks, you're done!
