One of the difficult things being a Vocations Director is that we often meet people who desperately want to be a priest. It is all they have thought about for months or even years. In some cases they believe themselves to have received 'visions' telling them that they are called to priesthood...
Initially individual has to discern whether they may have a vocation but ultimately it is up to the Church to decide. The call to priesthood is a call mediated through the Church. In the ordination Rite the ordaining Bishop asks of the priest presenting the Candidate: "Do you judge him to be worthy?" and the response is given "After enquiry among the people of God and those responsible for his formation, I testify that he has been found worthy". Only then does the Bishop 'call' the candidate to 'priesthood in the presbyteral order'.
What is difficult when someone clearly does not have a vocation is not the fact of telling them that. In fact it is a matter of justice not to lead someone along a path that has no future. What is hard is sometimes telling them in a way that they understand what you are saying (especially if they've received a 'vision') and also in way that is appropriately gentle and encouraging so that they can see that they do have a vocation to service in the family of the Church but that it doesn't lie in ordained ministry.
Why mention this seemingly negative problem? Because it brings out once again the need to allow the Church to look at the possibility you may have a vocation. Don't keep it to yourself, until you are 'absolutely certain'. Perhaps, if left to your judgement alone, that certainty will never come. At some point you have to allow the Church the possibility of confirming or otherwise the signs of a call you perceive within you. That's why it's important to contact your Vocations Director and begin chatting with him on a regular basis.