Is there a way to differentiate between a cold sore or impetigo inside nose? get occasional cold sores on outer edge of nostril. dangerous to brain?

See below: Yes, differentiation can be done with cultures of the fluid looking for bacterial (staph/strep) or herpes. As for the risk to the brain, the herpes viruses are already in the spinal cord and brain and comes out and manifest as cold sores or shingles, etc. For many reasons. Unless, you are having severe headaches, neck stiffness and pain, which could indicate meningitis, no need to be worried.

With impetigo: You get honey crusting sores and very contagious. Best treated with bactroban (mupirocin) / Mupirocin ointment. Need to heal the mucosa to stop the sores completely.