How do I know I am suitable for Master of Applied Maths?:
I suppose you need to enjoy it. I deal with a lot of tedious algebra, a lot of programming to solve equations. Specific to applied maths, often you have to think physically about the problem, like what sort of physical mechanisms are going on, what is driving the system, what are the different scales relating to different processes going on, what do the parameters mean physically. You have to like doing all of those.
Should I take GRE math subject test to find suitablity with maths?
Personally, I wouldn't. But why not pick up a few introductory texts and read through see if you like it.
Should I take math course to find suitablity with maths?
If that is an available option, that sounds like a good idea.