A clear job they say looks deeper, but there are certainly nicely done single stage jobs that look plenty deep, so it could be hard to tell the difference just by looking.
Only way I would know for sure is if you could sand a hidden spot or use rubbing compound on a spot(if you could sand or compound a singlestage without affecting the color can be a risk, and a metallic would be a no go for sure)
.A single stage with no clear over it would turn the sandpaper, water(wetsanding) or compounding pad color, since you are hitting the pigment. A single stage a very thin layer of clear resin actually does float to the top and help protect the pigment, but would require very little to be through it, Clear when sanded will be white. So it still would be hard to tell for sure if the color is white.
I'd hate to think any reputable painter would screw a customer and charge and tell them they are getting a base clear and then spray a single stage to pocket the amount of money difference between the two, and risk ruputation and future work. If I was working at a place and found out they did something like this to a customer, I'd be walking. No decent bodyman or painter would want to be associated with a place that did something like that either. But I suppose there are a few shady characters out there. I think most would prefer to shoot a base clear anyways in most cases, because along with it often being easier on them to shoot (especially metallics and pearls), and also if a mistake happens to be made to repair, and most colors and metallics is going to hold up to uv longer, and metallics are going to look better with a clear over them.