How does the official determine whether a receiver is eligible?

Answer 1

Usually this is only an issue when a lineman or other unusual position player becomes eligible for a play or two (think Mike Vrabel for the Patriots). The uniform jersey number ranges differentiate positions (QB's 1-20, RB's 20-40, WR's 80-90 and so on, though there are many exceptions), and usually the guys in the 50s through 70s are ineligible, these guys are linebackers and offensive/defensive lineman.

When the coach wants to make one of these ineligible guys a receiver for a play, he has to set them up off the offensive line as well as tell a referee before the play begins that they are an eligible receiver, sometimes with so much as the raise of a hand. I believe they announce this on the PA system right before the ball is snapped, in the form of "Number 66 is eligible."

Answer 2

Yes, diamond is correct.

Obviously, the two outside players (of the seven players who line up on the line of scrimmage) are eligible, as well as other recievers and backs who line up off the line...

But when a player who is not normally eligible (usually a tackle) wants to be eligible, he has to inform the referee, who in turn informs the defense. They can also announce it to the crowd, but the main thing is to inform the defense.

And players are indeed limited to the numbers they can wear, depending on the position they play.

Answer 3

The player must announce himself to the referee as an eligible receiver.