How would you handle this?
 

2-man mechanics. I'm L and partner calls a foul on a player who is attempting a layup. I'm 90+% sure that the shot is missed, but I'm L, so I'm not really looking. Partner signals 1 shot. I trot over to him and ask whether the ball went in; he says yes. What, if anything, should I do at this point? What if I were 100% sure that the ball didn't go in?

1) Signal a good basket. You should ask / be told this information while you are still at your spot or on the way to the table.

2) How could you be 100%? You weren't watching. ;) but if you are, you have to talk with your partner and decide who needs to visit the optometrist. You could also ask your other game officials at the table.

I believe that to make any ruling, you must be 100% sure of what you saw. If you're only 90% sure, you have nothing, and you should recognize that, and do better on subsequent plays.

If you're 100% sure that the ball did or did not enter the basket, and your P signals otherwise, go talk to him/her before they get to the table, or in the rare case: immediately after s/he leaves the table. Tell them what you saw and if they still insist, suggest that they consult the table crew. Alternatively, if you're the Referee on the game, and because the two officials disagree on whether the basket shall count, you can exercise Referee authority and rule that the basket counts or does not count.

On a side note, I would not ask your P if the ball went in. Instead, I would tell him what I have, and quite possibly why. By asking "if" the ball went, you are saying that you did not know. And I was taught that in 2-person, the non-calling official shall continue officiating, including knowing if the try was successful or not. Often officials will miss the gentle Team A rebound-tap that sends the ball into the hoop. The tap ensures that the basket cannot count.

I did continue officiating, and since I was L on the play, I wasn't responsible for knowing whether the ball went in. That's T's job. I just caught a pretty good look in my peripheral vision.

Partner as T called the foul---not me---so it's not up to me to signal and I'm not on my way to the table.

Happens a lot during 7/8G games! ;)

I think you answered your own question when you said, "I'm 90% sure it went in" and your P said "yes".

I wouldn't know! :p

Thankfully, as of late, I'm recalling it from memory......:)

My mistake. In that case, all you should do is get ready for the FT.