Two sides to this. There is what is actually a pro, and what the general person thinks of as a pro.
To a small kid at a resort, there might be a guy that is super sick, and sponsored head-to-toe. That kid (say 11-14) might say "wow, that dude is pro."
In a sense, sure he's pro. He's sponsored, he's good, he wins contests and he's a hero where you're at. You base your buying decisions off of him.
Of course, from a meta scale (like to a magazine, industry member, etc) this is usually referred to as a 'local pro'. We're noting that they aren't actually paid to ski by anyone - most often they're just being hooked up with the local rep's product and not even officially part of the company. That doesn't make them less dope, but it doesn't make them have clout on a national or international scale.
So is he pro for real? Sure... if you want to call him a pro and he feels like a pro.... go for it.
The next level is a 'real' pro. The simple defining factor her is that you are paid money by a company to wear their product. Even if the money is total shit, you have a contract and you are given this money in exchange for nothing other than wearing their product. If they give you money to work a demo day, and you are 'sponsored' then you are a local pro at best, not a pro, pro. YOu gotta get paid to be a 'professional' which is the pure definition of the word.
As well, most of the time a 'real' pro would have appeal on a more national/international level (ie partaking in popular ski media vs. just winning local contests) - and thusly would have a contract with head office of the company. This isn't the firm rule though - there are smaller local riders that might be paid by the Canadian arm of the company, but because they have so much appeal where the're from it works out.
Now, having said all of that I'm sure there's about a gazillion guys that are in between. They are in movies, shots in magazines, get ads, win contests... and nobody pays them a god damn dime. In this case you can probably use whatever term you feel like. You can say 'Pro' because that is what you feel is the correct terminology for them, but in the same breath you can note that they are in fact still 'Am' because they do not receive money to ski regardless of how good they are.
TL;DR - You're pro when you get paid to wear someone's product while you're skiing.