Frames are frames (in the entry level ranges anyway) but you can tell a bike shop quality bike from a *mart bike many times by looking at the components.
If all you see is Shimano, then it is probably a lower end bike, using components from Shimano below ther named level. If you see Shimano components (derailleurs) with names like Tiagra, Deore etc it is more likely to be decent quality. Usually, I think SRAM is reasonable.
To figure out where in the line-up something is, search the net (and the forums here) to find the hierarchy. Usually the higher in the hierarchy the components are, the better the bike. While someone might put good components on a low end bike, it is not very likely.
Most CL ads, at least around here don't list components, so once yuou get there and confirm these points, just check the bike mechanically to see that everything works, nothing looks bent (like a front end collision bending the fork/frame) and the wheels spin pretty straight.
As far as brands, look for anything offered by bike shops like Trek, Canondale, Giant, Specialized and many more. Some brands have big box level bikes and decent entry level bikes, depending on where they were sold, like Diamondback, Schwinn and others... I don't know what to tell you there.
On CL, don't believe how much they paid for the bike unless you can confirm it, nor that they know what they are saying when they talk about speed, weight, or quality. I have seen $99 Walmart bikes on CL used for $250+
And you can always ask here, but if it is a really great deal, it may not last long enough to get any good feedback... and was possibly hot anyway.