not sure at a hub, but in SJU, there are 2 UA flights -- IAD/SJU/IAD and ORD/SJU/ORD -- which don't usually overlap on their ground-time in SJU, and leave from the same gate where they arrive, so it's pretty easy to know.

even easier someplace like BKK, where there's exactly 1 UA flight in and out each day.

and yes, it was nice on my MRs to SJU knowing that i would never misconnect in SJU.

on one of the runs, i missed a golden opportunity though:

had flown the LAS/SFO/ORD piece of an LAS/SFO/ORD/SJU/ORD/LAS MR. had turned off my phone while sitting in the plane in ORD, and the pilot announced that he the plane needed 2 tires changed, and we'd be delayed an hour. my thoughts were, "no worries, i can't misconnect in SJU." my thoughts should have been, "turn on your phone." a GS agent had called and left me a message saying they'd re-routed me ORD/LAS instead of ORD/SJU/ORD/LAS to make sure the mechanical issues didn't result in me making it home that night. i could've spared myself 1/2-day of travel and then asked for original routing credit... darn.

I did a double trip MR run to Hawaii 3 weeks ago (Fri-Sat and Sun-Mon). Was recognized by a flight attendant from the Fri outbound flight who was working the red eye Sunday night. She commented I'd had a quick trip - didn't have the heart to tell her that I'd actually been home on the Saturday and flown back to HNL again on the Sunday!

Was fun telling a nosy PAX at ORD who was giving me 50 questions about my trip when asked how long I'd ben in Hawaii. "An hour"... he thought for a minute, opened his mouth to ask question #51, but then shut up :)

Another fun part of the trip was spotting fellow MR'ers... one guy got off the plane in MKE and looked lost... 10 mins later he got on the MKE-ORD flight on the same metal he'd just flown up on. I was laughing to myself knowing what he was up to!