You need to get tested by someone who has trained in the appropriate power. A lot of people won't do it because of the risks, and you probably aren't Force Sensitive anyway.
The best advice is to play your character as if you aren't FS, and get tested if you get the chance.
____________
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
By "risks" I refer to the generally screwed up and completely imbalanced Out Of Faction Arrest & Execute (usually abbreviated to OOF AE) rules. You can check them out in the Combat section of the Rules pages. Basically, it makes it very risky for anyone to be in the same place as someone they don't trust completely.
____________
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Pretty much none, unless you happen to be good friends with one. Basically every new player wants to be tested, and there's a much higher demand to be tested than there is a supply of testers, so you're probably out of luck.
“so what about some1 in the same faction as you?
from reading the rules it sounds like he would still need to trust you.”
It depends where you rank in the faction compared to them. Usually if you outrank someone, you're safe from them. Of course, they can also easily change faction and kill you under authority of their new faction. See the GNS for recent examples of this. It's hard to trust anyone, especially when a number of factions have been robbed blind by apparently "loyal" members, even leaders, with the appropriate privileges.
“under what conditions would a jedi/sith try to tell you if your force sensitive?”
When they feel like it, basically. Some of them are friendly with certain factions. Some of them are paid. Some of them are just nice. But mostly your chances really aren't good.
____________
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."