In fact, there is a way of determining the chance. Check up the rules and find the formula to calculate whether you are FS or not. At first level, as a Gand, your probability will be 6%, which is very, very poor. However, as you increase in levels, this probability will increase every 4 levels (I think), and you will have more chances - like 2% or something, which isn't really that great.
So the best thing you can do really is forget about the whole Force Sensitive thing, and play for fun until you get a change to be tested - e.g. the faction you're in has a Jedi coming in to do it. That'll mean you're playing the game to enjoy it.