. How does one differentiate the zeal for the faith and the call to the priesthood?
I don't think this can necessarily be done at first. Life in the seminary takes a long time for a reason and part that time is discernment. If a man waits a year after he converts and then goes to the seminary, that will probably be about 8 years or more before he is ordained a priest. I think that will be plenty of time for discernment and to receive help from spiritual directors and priests.
The advice I give to converts is that they should follow up their desire for a vocation and see where it leads (its not like one can enter the seminary overnight anyway). Being in the seminary for a few years will be a positive experience and make one a better man (usually) so that won't be time wasted if you go to the seminary and realize that you don't have a vocation.
I think this is much better than the advice I have seen given which states that a man should wait a few years to see if it is just convert zeal. Many men realize before they are even the seminary (or a monastery) that they don't have a vocation. But if by the time they can enter the seminary they still think they might have one, I don't think anything will be lost if they do eventually leave the seminary, but something might be lost if they wait too long to find out if it is merely convert zeal.
I hope this rambling makes sense.