A1 passes ball to be A2, A2 catches the ball while in the air, B1 tackles A2 making him land out of bounds. A2 MIGHT have landed in bounds if he weren't contacted.

How do you make this judgement call if A2 would have landed in bounds or not? Who's side do you lean towards if its close?

Well of course the rule you are referencing is:
2-4-1 A catch is the act of establishing player possession of a live ball which is in flight, and first contacting the ground inbounds or being contacted by an opponent in such a way that he is prevented from returning to the ground inbounds while maintaining possession of the ball.

This is a difficult one as you only get a split-second to evaluate it. Do the best you can based on what you see. I don't have much to help you there.

But here are some guidelines printed on page 21 of the official's manual.
Incomplete pass or fumble - incomplete
Forward or backward pass - forward
Kick or pass touched or not - not
Catch or not - no catch
Passer has thrown or fumbled - has thrown
Touchback or safety - touchback
Fumble or dead ball - dead ball
Apparently injured or not - injured
Apparently unconscious or not - unconscious
Accidental contact, touching, or kicking or intentional - accidental
Legal block or clip - legal
Block above or below the waist - above
5-15 face mask - 15
5-15 kicker or holder - 15

I try to watch the arc of the jump and I try to estimate where he would have landed and I can usually get a pretty good sense this way. I don't know if I've given much thought to "when in doubt" but "when in doubt, the pass is incomplete."

When the receiver is forced out (in NFHS), I give him the benefit of the doubt on the reception. Only if there is serious doubt.

Rich

Snake eyes it is almost impossible to give you an answer based on the information that you have provided. Did the contact from B change A's direction? For instance, if A's momentum is toward the goal line and B's hit moves him sideways out of bounds, I probably have a catch. If A's momentum is toward the sideline and B's hit is from the side and does not change his direction and he lands out of bounds, unless I am absolutely sure he was coming down inbounds I have it as an incomplete pass. If A is hit from behind and he lands OOB I have an incomplete pass. This is a situation that you almost have to see to give an informed opinion.

I'm not asking if it was incomplete. I'm asking what's hte best way to judge this situation and who gets the benefit of the doubt.

NCAA rules - receiver only gets catch if "carried" OOB so that is simpler.

NF - I think the best way to judge is if the contact changed the direction as noted by Keystoneref. If the contact mostly adds to the direction the receiver was already moving...no catch. Otherwise give benefit of doubt to the receiver as noted by Rich.

REPLY: For NCAA, CKref has it right--the receiver must be wrapped up and carried OOB. Otherwise, the pass is incomplete. For Federation, I suggest that you look at the Federation Case Book p. 13. Look at the note under the title "Catching the Ball." It says that the only way that you can even entertain the idea of a legal catch is if the defender's contact is such that it stops the receiver's forward progress--regardless of whether or not you judge that he would come down inbounds. Then take a look at Play 2.4.1 Situation H which codifies this concept.

Position, position, position....
 

to call this one right, you've got to be in the right position, otherwise it's a best guess.

So, in addition to what others have said about the arc and the relative position of the players in relation to the sideline, I believe the most important mechanic is to position yourself straddling the sideline, still as stone, 10+ yards away (if possible) to get a good view of what is actually happening.

If you're like me, sometimes I get "happy feet" and get too close to the play and need to stop moving to make a good call. Darn it!