Knowing if you can actually fill your needed role is important. But you have to be honest and realize that just not everyone is open to allowing off-specs into raids. And while you might get into a raid eventually, are you actually a regular player? Are you someone who is actually contributing to the raid as a whole?
The whole goal is for you to play an active part in your raids daily successes, not sitting on the sideline hoping for a pity invite. So really you have to ask yourself from the outset if your guild and raid leader are open to giving you an honest shot.
One way to go about doing this is to get support from key members of the guild. Class leaders and officers can do a lot in getting you in on raids. Of course, it also doesn't hurt if you are an officer or guild leader yourself.