Sorry to hear that you have joined the ranks of Rosaceans with a seb derm overlap. I also had perfect problem free skin (save for being an easy blusher), that was at least until my early thirties when things literally changed overnight. I spent nearly a year torturing myself over what caused it, what I did or didn't do that lead to it, etc, etc. Please don't do this, the fact is that doctors really are not sure what causes either Rosacea or Seb Derm to develop in some people and not others (good theories out there about both, however these theories have not lead to "cures" and I find for every study about causes I found saying one thing I could easily find another saying the absolute opposite). You are better off putting your energy into finding a good dermatologist and a treatment that works for you.
I wouldn't get too hung up on trying to figure out what portions of red skin are Rosacea and what parts are seb derm, trained dermatologists have difficulty as the conditions overlap frequently. If you have Rosacea with seb derm you will need to treat both to be effective, as both are inflammtory skin conditions, treating one and not the other is going to hinder progress as your skin will remain inflamed.
1. Seb Derm redness will respond to treatment. So after a few days of using one of the classic treatments (or new ones) for seb derm - that is a mild steroid such as hydrocortisone with or without ketoconazole, or one fo the new topical immunomodulators such as Elidel or Protopic, the redness should correct, if it does not it is more likely permenant redness caused by Roseacea. It bears noting also that Seb Derm, although a persistent and god damn annoying condition will wax and wane, so even without treatment the red patches get better...eventually (it is very persistent if not treated and can "hang out" for years). I would also just add here, don't get suckered into using just ketoconazole cream, I would bet dimes to dollars it won't work, and the treatment time is a few months. That is about the only thing every one of the derms I have seen agreed on (Keto alone for seb derm as an initial treatment won't work), except the first one who wasted two months of my life putting that crap on my face, which not only felt bad (the base of Ketoconazole Cream, that is the crap they put in it besides the active medical agent, sucks, makes you wonder who is designing this stuff, someone should have told them it gets applied to human faces, maybe that would have improved it).
2. Rosacea redness has two stages, transient redness caused by vascular flushing. In "stage one" redness the "red patch" should return to normal colour after a short period of time (usually ranging from minutes to a few hours). Over time however this redness will enter "phase two" where its just permenantly red (most commonly in Rosacea this occurs in the nasal folds, the cheeks, the nose and the chin, however it can occur on the forehead and even other areas of the body such as the chest - but this is rare apparently). So if the redness never fades and doesn't respond to seb derm treatment it is likely permenant redness caused by Rosacea vascular flushing.
I highly recommend that you try and read as many studies as possible on Rosacea and Seb Derm. One consistnet view appears to be that Rosacea is almost always the dominant condition so if your dermatologist wants to treat you for seb derm only I would discuss this with them. If they appear confused or are unaware of these studies I would get a new dermatologist, they lack the knowledge ot treat you properly. I went through 6 dermatologists and 2 years of hell before I lucked out and now see one that is on the RRDi Research Board for Rosacea. He not only confirmed the above but advised that it is likely in the next few years that seb derm will be added as one of the possible phases/classifications of Rosacea as it is such a common overlap (occurs apparently in over 20% of Rosaceans).