There are indeed lots of free tests that you can take online. I would encourage you to take a multi-pronged approach, as I don't feel one test is magical (certainly not offered free online!).
As with the students/clients that I work with, I'd recommend the following sites to take the assessments and learn more.
A personality test (MBTI), learning style(s), multiple intelligences, and career interest inventory can provide a well-rounded look at how you gather, perceive, and process information and communicate. I find that, together, I can better assist students/clients find their calling.
I hope that this information will prove useful to you as well! Good luck to you on this journey - and don't forget to enjoy the ride!