PlayerA casts a Ruin Processor and, as he puts it on the table, announces "When this resolves, I will put your Mountain from Exile to your graveyard and gain 5 life". PlayerB is aware that Ruin Processor has an “on cast” trigger rather than a “enter the battlefield” trigger. He says “resolves, but at this point I think you missed the trigger already.” - They don't agree on that, therefore call for a judge.

At Competitive REL/Professional REL, how can the judge distinguish Out of order sequencing from a missed trigger here ?

I don't see this as any infraction or OoOS. For all an observer knows here, player A may be referring to the trigger itself resolving.

“Player B, don't be a jerk. Trigger resolves.”

d:^D

Out of Order Sequencing, if applicable to a situation, will override what would otherwise be considered a Missed Trigger. Although, in this case, I don't think this would be a missed trigger even if we didn't have the Out of Order Sequencing section of policy.

I agree with everyone above triggers like that are really only missed once significant game actions have been made, and in my mind a spell resolving hardly qualifies.