The presence of a 12v output has nothing to do with it.
If someone honestly doesn't know if they're selling an inverter gen. or not then they're likely not.
I'm not aware of any inverter gen. larger than about 3,000 watts except Honda and Yamaha. Most inverter generators tend to look all "closed up"; that is, they have side panels and look a lot like a box. The regular type generators are often referred to as open frame. They have a frame with the components mounted to them with lots of room in between where you can "see thru" the machine.
Your best bet is to simply get the model number and look it up online. Any seller that can't give you the model number is not someone I would deal with.