What's the best way to know if my gums have receded to a dangerous point?

Receding gums: Look in the mirror and if you can see the root portion of the tooth and it is a lot longer than the nearby teeth you need to see periodontist since advanced recession also means your supporting bone is receding and if too far along the tooth will become loose and be in trouble.

See a dentist: There is a faint line between the attached stronger gingiva and the thinner mucosa. Only a dentist could tell you how bad your situation really is.