For those reading along at home, none of those distinguish MRSA from a normal staph infection.
The primary distinguishing feature of MRSA vs regular staph aureus ("MSSA") is MRSA's resistance to oxacillin/methicillin. While you can find research papers distinguishing MRSA from MSSA on clinical grounds, I'm not familiar with any clinical distinguishing feature that is used in practice (aside from sensitivity analysis or PCR, which are laboratory techniques, not clinical ones).
What you are describing could be necrotizing fasciitis, and although MRSA is a common cause there are many classes of pathogens responsible. Any of them can constitute a medical emergency, and if not diagnosed and treated quickly a surgeon may need to butcher you to save your life. I've been in the OR for a debridement and you do not want to be that patient.
If you have pain way out of proportion and darkening skin (or any rash for that matter), that's not the time to play House. Call your doctor if you have one, or just go directly to the ER.
1. Intense pain that extends beyond the boundaries of the visible irritation. Pain that is much more intense than a normal pimple or ingrown hair.
2. Distinct rash (as far as I understand the rash is caused by the specific toxins MRSA generates)
3. Necrosis of the skin. MRSA will produce blackened dead skin which many people mistake for a scab.
That, and I had MRSA years ago and had it diagnosed by a doctor, so I was already familiar with the condition.