Good indicators that your abscess is actually a fistula are: you pass stool through the abscess
opening, or you pass gas through the
opening. If either of those happens, it's almost a sure thing.
Otherwise it's difficult to determine on your own. If you have an abscess that continues to drain pus and is taking an exceptionally long time to heal, then you may want to schedule a visit with your doctor. A doctor can try to determine if it is a fistula by inspection and a bit of probing.
Abscesses definitely can itch as they heal. However the itching can be any number of things - including hemmerhoids (assuming peri-anal abscess?) or inflammation making it difficult to keep things clean. Nothing to worry about, but I would avoid itching because it can irritate the area and if the itch is due to hemmerhoids you can damage the blood vessels.
Average healing time for an abscess varies a great deal. My own abscesses have cleared up in about 5 months, 3 months, and 4 months respectively. Many factors affect healing time: location, size, degree of crohn's activity, nutrition, etc.
Generally in people with Crohn's, a fistula will not heal up on its own - though it is possible. Usually a seton is inserted to prevent the creation of an abscess and allow the fistula to continue to drain when it needs to. Biologics have about a 60% success rate in healing fistula after a couple of years (you can look up the exact stat if you are interested). As far as I know, only biologics have been reported to heal fistula.
A fistula that doesn't heal isn't the end of the world though. In most cases there is no pain, and minimal to no drainage after some time so it just becomes another quirk in the aesthetics of your bum. :)