Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services for sudden weakness or numbness accompanied by facial drooping, difficulty speaking, confusion, loss of balance, or a severe unusual headache.
These can be signs of a stroke rather than an isolated mononeuropathy.
Emergency help is also needed for weakness affecting breathing or swallowing, or for rapidly spreading paralysis.
And for anyone, someone who:
Seek prompt medical assessment for new foot drop, facial weakness, rapidly increasing weakness, or symptoms following an injury.
New numbness or pain under a cast, splint, or tight bandage also needs prompt attention, especially if the limb becomes pale, blue, cold, or swollen.
And for anyone, someone who:
Arrange a routine appointment if you have persistent numbness, tingling, nerve-like pain, or weakness in a specific area.
Earlier assessment is particularly important if symptoms are worsening, interfere with walking or hand use, wake you repeatedly, or are accompanied by muscle wasting.
Every sign above is quoted from the sofidex article on Mononeuropathy. If you are not sure, err upward — it is always reasonable to be checked.