I have never been called to preach, I don't think!
That being said, I think one knows in their heart. One might seek affirmation, but it really is between you and God. I think that some have been called only to be turned back by their pastor for one reason or another. I know a pastor who told a young 21 year old that he was not called to preach. It turns out that his own son was 21 years old, and the pastor was grooming his son. Well.....the 21 year old kid left and started his own work in his home. Within five years the kid was running over 400 in attendance!
I'm not against pastors giving you advise, but the only way to know for sure is to get out there and preach and see what happens.