Take a breath and try to remain calm.
Do you regularly check your oil? If you don’t, then you should start. (I’m one to talk. My oil level hasn’t fluctuated in my 2004 Camry in six years, so I don’t check it any more, but I should anyway.) If you know the variations in levels on the dipstick, between “warm” and “cool”, for example, and between “full to the mark right after an oil change” and “down a quart”, then you can’t be flim-flammed. So get used to checking the dipstick; that’s what it’s for.
For normal readings, it’s best to get into the habit of looking at the dipstick AFTER each fillup with fuel. That gives the engine a few minutes to cool and for all of the warm oil to drain into the pan after your drive to the filling station. It’s best to check when the car is cold and level on your driveway before you start each morning, but that’s not always practical. When you’re stopped for fuel anyway, then checking the oil is only a two-minute addition of time. Not a major handicap.
Did they show you the oil on the engine that they were concerned about? An “oil leak”, as you expect, would show up under the parked car as it rests. “Leaking seals” are another matter. That’s where the hot oil is forced out of the engine – potentially at any point where a seam exists and is currently sealed with a gasket or other seal of some sort – and can go up, down or sideways. This will be noticeable on upper engine surfaces. A well-maintained engine should not be excessively oily. There will be some oil and some films of road dust, mud spatter (maybe) and other grime, but not a heavy, greasy-looking and generalized oil accumulation all over.
Don’t rely on “check engine” lights entirely. If that’s all you do, you’ll learn the reason why those are called “idiot lights”.