How would I know if God were calling me to be a Jesuit?

Being able to talk about that with a spiritual director is also helpful.  You might also find that the idea of being a Jesuit brother or a priest simply fills you with considerable peace rather than anxiety.  Again, if this is talked through over a period of time with an experienced spiritual director, some light can be shed on the source of the feeling.  Other external factors can come into the process of decision-making; for example, someone with poor health may simply have to accept that the demands of life in the Society may be too much for him.

Eventually, of course, you will just have to decide whether or not you are serious about joining the Society - there are no cast iron certainties about this, just as there are none about other decisions you might have to take in life.  To some extent, the period of early training as a Jesuit can show up weaknesses in a poorly-made decision or help confirm a well-made one.