Basically, you turn "Pro" once you start making money. But that is just the general term.
I would describe it much like EgsEMT did.....
1. Consistant work (the people get the same level of work that they see when they view your portfolio)
2. Terrific business skills (Giving work "away" is easy. Selling it is hard. Be able to sell your work for what it is worth, and be able to sell it against like experienced, like priced competitors)
3. Making enough to live off of (Could your photography business replace your "old job", moneywise?) ...I know part timers who can answer "yes" so it's not a full time vs. part time question.
4. Great customer service (do you have the time to dedicate to giving your clients the very best you can?)
5. Are you charging enough to make a true profit? (Is your gear payed off in cash by past jobs...advertising...paperwork...products...and at the very least your time?)
6. And most importantly are you displaying professional results? (This will vary depending on the amount of experience level, equipment level, customer service level, selling ability, pricing, and final product.)
7. Are you paying your states sales tax and carrying the required liability bonds? (Anyone can buy a camera and put up a website, but not everyone is paying their sales tax, which is easily disclosed, nor the state required liability bonds. Believe me, there is a shark fest on the sales tax alone....If a person is making money off of photography, and not paying the sales tax on their state, it's an easy breezy search to find out. The competitor photog who turns in the non paying photog gets a "deal" on their next quarter. Don't confuse this with Federal Tax. It is different. I've seen a ton of photogs go down this way. Liability bonds are a different matter. My state requires "real" photographers to carry a 1 million dollar liability policy. It costs about $500 a year. If you book a client, and don't have the required bond, you can't work the venue. If you can't work the venue, you can be sued by the client.
So basically, there is a lot to think about. You can be a pro based on earning abilities, but "going" pro is much more. It's not just about having a camera and taking photos. There is a lot of finacial and legal stuff to be attended to. There is a lot you need to do to cover your butt, and your client's butts.
I wish you well on your endevors. I honestly do.