I have an old well on our property (drilled around 1920's or 1930's and also 5 new wells just drilled in the past 4 years.
I would be surprised if you could buy any property that already had gas wells on them and they would sell the mineral rights with it. Around here, most mineral rights continue on down from generation to generation.
They do not have to add a "scent" to the gas. As they are drilling, and if they hit gas, you can smell it.
Most places that already have gas wells on them (and again you would be lucky to be able to get the mineral rights too) depending on how old the gas lease was when drilled will tell you how much free gas you get. Most old wells the lease states about gas for the house, perhaps for a chicken brooder house, etc. and does not state an amount. As long as you don't go hog wild and put gas in your garage, barn, outhouse, etc, the gas company won't mind.
With wells already on the property, you will have to see who actually owns them. With the old wells and leases, the person got free gas and usually a royalty of $xxx per year. My grandparents have a lease that gives them $50.00 / year. The gas company is making money hand over fist on these old leases because there was a set amount and it never increases. (I have heard where you can take them to court to get current rates, but unless the gas company actually comes in to change something with the well, the old lease written will stand up in court.)
Most times with the old leases, the gas company owns the wells and the property owners get free gas, the mineral owners get some sort of royalty. However, in our case, the old well that was drilled way back when, got sucked down so much that it wasn't "worth it" for the gas company to keep it. They sold it to mineral owners for $1.00. You say, what a deal, a gas well for $1.00 plus all the gas you can use right? Well, maybe. If that well happens to dry up and no more gas comes out, it will now be your responsibility to "plug" it. (Using cement, stones, etc. to fill the old gas well hole and prevent water contamination etc.) This is not a "do it yourself" job. It will have to be hired done, and I hear can cost $10,000.00 - $20,000.00 to do it. The gas company has made out again in this scenario. (Incidentally, that old well is still producing gas today.)
For any new wells drilled nowadays, (and if it's a new gas lease), the mineral rights owner will get free gas (usually anywhere from 100,000.00 cubic feet per year and up) along with 1/8 of any gas sold. These royalties are considered "income" and will be required to pay income tax on them.
If you don't understand what I have just told you or have any other questions, just ask!