This is a great question and the answers here (from Bakmoon and Suminda) are wonderful. I'll complement with something from my personal experience.
"how can one determine ones decision to ordain, whether it's a strong volition or not"
It may be that going forth ultimately is the single thing one can do to know the true nature of his disposition. But since disrobing may come with a lot of practical difficulties, taking precautions is very valuable: become used to a life without luxuries, familiarize with the monastery, monks and retreats, etc.
But giving the ear to advices may be troublesome, depending on who one seeks out to get advices. There is a common discourse that people looking to become a monk often hear in the west: "you are running away from [blank]".
It may be more articulate than that, when it focuses on debunking the romance of becoming a monk: the idea that things will be so beautiful, you will have 100% of the time to meditate (or do prostrations, or read suttas, or perform all rituals you fantasize about, and wisdom will come quickly, and your teacher will be so and so, etc).
Or it may focus on the idea that a monk's life is easy and therefore, not very fruitful for evolving one's practice: "the lotus grows in the mud, so you should do your practice in the real life, not in the top of a mountain", they say.
On one hand, these are very unhelpful advices, and very disconnected from what monastic life is, and from what buddhism is -- at the minimum, from this rhetoric, no one is suited for monastic life, or should become a monk ever (it is also a little disrespectful, drawing an image of monks as cowards).
On the other, there is a risk of fantasizing with a romantic idea of being a monk and there is a risk of using the going forth as an excuse. I think the best strategy for the first is what others have said here, try to become familiar with monastic life, in order to dispel any fantasies you may have. The antidote for the second is to try to identify if the intention of going forth is the going forth itself, and not just the relief of not having to deal with "[blank]" anymore (its ok to feel good about not having to deal with "[blank]" anymore, but I think this should not be the reason for going forth).