You cannot assess as much as a Career Counselling professional, but knowing the theory and interpretation of Career assessment test and report, you can do some self assessing exercises.
To understand your interest, analyse the kind of task you enjoy performing on a daily basis eg. if you are more inclined towards artistic kind of activities, social activities ie. helping others, physical kind of activities, business or administrative and so on. Second parameter ie. Aptitude meaning capability, try to understand if you are good in maths, drawing, verbal reasoning, etc. Third parameter ie. personality meaning trying to understand if you are an introvert, extrovert, passive, assertive, etc. Do a self analysis of all these 3 parameters, categorise yourself and co relate them to matching careers requiring the relevant parameters. Sounds difficult, morover incorporating all the parameters of so many careers sounds like an impossible task.
Allegiance Educare's proprietory algorithm and Test Engine, calculates more than 5000 combinations of more than 500 careers to generate an automated report with shortlisted careers.