How to test whether I am suitable to pursue mathematics? I am a high-school student. I have developed a recent affection towards mathematics (especially proofs), although I was always into logic (I've been a programmer since the age of eight). Now I am worried whether I will succeed in mathematics as a career. I have an IQ in the 99.7%ile in India and always score approximately 150 in IQ tests. (I know IQ is irrelevant.)
To judge whether I am capable enough to be a mathematician, I have set myself a challenge: if I clear the entrance exam of ISI, only then shall I pursue mathematics. (Sample ISI exam questions.) I want to know from current math grad students, professors and teachers whether this challenge will be good enough to test whether I have the ability to pursue mathematics as a career. If not, then how can I judge myself?
I don't have access to olympiads, though the subjective questions of ISI are considered on par with InMO, and clearing the exam is also considered a big achievement in India.

With all the respect, I do not think this is the right question to ask. You can spend your whole life wondering whether or not you are able to do something, but the only way to actually know it for sure is to go and try.
So I did - it turned yes.
Good luck!
