How do I know whether an airline seat upgrade at gate was free?

If you didn't sign for any payment, it was a free upgrade, period.

Anything else would be impossible to prove for them - how could they ever prove that you agreed to pay X for it, or that you even asked for an upgrade? If this were possible, they could just charge anybody they feel like an upgrade fee.
Also, they wouldn't have access to your credit card to directly charge it either.

All this shows is that - depending on availability and situation - you can still get an upgrade simply for nicely asking.

I’ve flown a million miles in 10 years across 13 airlines. I’ve been upgraded hundreds of times at the gate and it’s free. You walk up, ask to be put on the upgrade list, they put you on it, and that’s the end of it. By the way, be VERY nice to the desk attendant. You can then get upgraded 1 of 2 ways. First, it can happen preboard. Just watch the screen, your name, and you either get upgraded preboard or not. The close outs for all boarding include all upgrades. Second, is post-board. Maybe 1-2 out of 10 times after I board, about 15 minutes later some attendant comes back to my seat, and let’s me know I’ve been upgraded and kindly walks me up to an open first or business class seat. Thats usually because someone didn’t make the flight at the very last minute. At that point the flight attendant with the boarding attendant will discuss eligibility and visual dress code they can see if the few who are in coach on the upgrade list, and if you look professional and were not a jerk back at the desk, and you don’t look like you’ll be a drinking problem up in First, you get the upgrade. I’ve chatted up dozens of attendants and pilots, and that’s the long and short of it. And no, there is no charge for the upgrade.

Airlines don't usually give someone a seat and send them a bill later. As you were not asked for payment, the upgrade was complementary. They do offer paid upgrades but in that case, they specify the cost up front. It will happen when a flight very full but not completely and more so for flights where the upgrade makes much more of a difference. When flying US to Canada, it is similar to a domestic flight and therefore the amenities of business class are often little more than a larger seat (or even just increased space between seats and an additional tray) but complementary snack and beverages.

It is highly unlikely there would be a share that you did not sign for, it opens for to many misunderstandings and liability issues.

Depending on your frequent flyer status etc there are normally many free upgrades to be had but I have found and also confirmed with people who work in the business that most of the time the people who work the gate dont go though the hassle unless you ask.

In a nutshell, we could say that if nobody asked you to pay at that moment you can simply relax because you won’t have to pay anything.

Airlines have decent CRM systems which suggest who is suitable for a free upgrading in first place. Since you’re saying you had no Frequent Flyer card with that airline I’m quite sure you got lucky.

They had free seats and they applied you that upgrading.

I’m still not sure how “upgrading” was that (another cabin or just an “XL seat”?) since we boarding operators use to justify “Operative Upgradings” or waived ones. They might just told you to seat at that seat and didn’t update the seat you had on the system, who knows.

Boarding times are so tight and clock is continuously ticking. Hope you enjoyed the flight!

Nah, I dont think theres a way they can charge you without quoting a price. You are upgraded when the flight is full and you can not be accomodated in economy, When the flight is close to empty and they have the resources in business class, when a person who has booked a business class seat but does not show up.