This has come up in our session and I cannot find the answer in any of my books.

How does a person with magical abilities find out which college they should enter?

I know wizards search out prospective candidates, but it doesnt describe the method after that initial meeting. Suddenly they are in a school.

This is only for Empire as we know other areas have different rules.

Even though I heard rumours of other countries wizards ending up in the colleges, there still must be a method of how,

I was thinking theres a College Enrolment Building were the candidates are sorted into their school, and this public building is known to one and all. As seemingly the Colleges themselves tend to be hidden.

Well, it's described partially. They'll know of the colleges and notice which powers they themselves possess.They might get that wrong however, and that might end with sigmarites and a bonfire.

The Jade Orders applicants tend to follow the leylines and find stones inscriped with instructions for the new applicants, and thus they are led to the order. I'm pretty sure the others are similarly "drawn" towards their respective colleges because they're placed where their particular wind of magic is very strong.

From the 2nd edition Realms of Sorcery p. 74:

"The personality attributes that allow Magisters to judge whether an applicant is best suited to their College are easiest to see in the young. As Aethyrically sensitive people grow older, and perhaps start manipulating the Winds of Magic subconsciously, it is harder to see a clear-cut path in regards to which Lore they possess the greatest skill for."

"A Magister confronted with an uncorrupted and magically sensitive young person who is not best suited to the Magister’s own Order is required by Imperial law and his oaths as a Magister to send or drag the applicant to a representative of the College he believes would be more suitable, along with a letter of introduction where necessary."

Realms of Sorcery also gives descriptions on typical traits expected of apprentices for each College.