The fact of the matter is that every Christian already knows how to discern their vocation; every Christian has the vocation of their baptism, which is to be Christ in the world, to love God and neighbor, and to stand for the faith and all else that is good, true, and beautiful. (The three properly understood come to the same thing.) All other vocations are merely secondary extensions of this. As Hannon says,
The Christian ought to make major life decisions as he ought to make all decisions: by evaluating how he can serve God, by choosing a course of action accordingly, and by having the courage to follow through and do it.
Here's a checklist on how to decide if your vocation is marriage:
1. There's no fundamental impediment to getting married.
2. You've met someone really great.
3. You think you'd like to be married to them.
4. They think you're really great.
5. They think they'd like to be married to you.
6. You could meet your responsibilities as a married person and they could meet theirs.
7. It wouldn't be an act of stupidity in general or a harm for yourself or the other person for you to marry them.
Here's another checklist on how to decide if your vocation is priesthood:
1. There's no fundamental impediment to ordination.
2. You are interested in being a priest.
3. You could fulfill the responsibilities of a priest without scandal.
4. You are willing to commit to putting other people's good above your own, and especially God above yourself.
5. It wouldn't be an act of stupidity in general or a harm for yourself and others for you to become a priest.
Of course, these aren't even universal; there have been arranged marriages and there have been times and places where congregations forced promising young men to be priests. But, again, it's really not that difficult to make decisions.
ADDED LATER:
Given this discussion, I should probably clarify:
(1) The checklists are not really intended to be rigorously reductive, since they are really intended to be checklists for finding out what one needs to know in order to make a decision and not algorithms for making the decision itself (I don't think any possible checklists would suffice for that). Taken this way I think that at a very general level they cover all that's usually necessary, barring special interventions by God, rare circumstances, and deliberate human perversity. I am afraid I was being somewhat sarcastic, though, in making the point that discernment of vocation is perfectly ordinary practical reasoning, albeit about a very important thing; and this no doubt obscured the intention.
(2) Strictly speaking it is possible to be Catholic and have both a vocation to marriage and a vocation to priesthood; it happens fairly commonly among Eastern Catholics (they only have the celibacy requirement for bishops), who are, of course, Catholics in good standing. And the discernment of both could overlap; I know at least one person for whom I think this was the case. It is, of course, not possible for Roman Catholics (and Eastern Catholics, to avoid confusion, avoid it in areas dominated by Roman Catholics), because of Roman canon law, except under unusual dispensations arising from conversion (usually from Orthodoxy or Anglicanism), which wouldn't be relevant to Catholic discernment. So the incompatibility is not a general one, but depends on the Catholic tradition to which one belongs and the canon law governing it. It is certainly true that if the checklist is not taken to cover this already under its weasel clause of "act of stupidity or harm" then it would have to be amended to include it as something you would need to find out if you didn't already know. This is a good example, actually, of how 'discernment of vocation' is concerned with practicalities. What one 'feels God is calling them to' may well be such a practicality, as one's own interest certainly is; but most of real discernment deals with things like whether it's legal or whether it would cause confusion, matters that require clear information and objective analysis.
