I'd say you have to be almost certain. In everday play, I call pretty liberally in favor of my opponent, even being told sometimes that I played out balls.
In tournament play, I call a little bit tighter, but my standard is "did I honestly believe it was out." I know that a ball that lands super close to the line can be a fraction in or a fraction out. It's almost a gut instinct (and years of having called my own balls and looking at similar shots over and over again) as to how I call it. I'm sure there are a few times when I honestly thought it was out, but it may have been in by a hair. I'm not trying to cheat anyone. I'm not saying, "it could have been in or out, so I'm going to call it out." I'm saying "I genuinely believed it to be out, there was something about it that distinguished it from balls that are just a fraction in," so I call it out.
IMO, the call has to come almost immediately. On non-clay courts, you can't really pause and then call it out. And, we've all wanted to. We've all had those calls where you kinda of feel that it was out, really think it was out, but just didn't get that good look or that automatic feeling. So, you groan to yourself and indicate to your opponent that it was in.
And, for me, balls that land parallel to me on the side lines are a really tough call. If I'm at the baseline and the ball hits the sidelines somewhere ahead of me, then it's fairly easy, but when it's straight to the side of me, it's a tough call. In normal play, when I'm playing against guys I know and generally trust as honest players, if I call that shot out, and I get a bad look, I'll just ask them what they thought and change the call.
Being a good line-caller takes honesty, focus, and experience. Everyone will make mistakes.