It's difficult. Rather than looking at the glitz, root around inside cabinets and cubbies, crawl inside the front storage compartments and look at plumbing and electrical wiring runs, and for sloppy and/or cheap components like bathroom faucets. Compare the frames and running components, including tires, from different mfgrs. to see if there are meaningful differences.
I knew I was not buying a top of the line FW, but here's just one example I can share: after determining I had a very small leak somewhere in my plumbing (new FW), I removed the access panel for the washer/dryer hookup and discovered one of the four screws holding in the panel was screwed into the hot water line! That's sloppy workmanship IMHO. Not only that, I discovered the PEX lines throughout the FW have numerous unneeded joints and fittings. PEX is designed to be flexible (to a point) and I could have eliminated up to 20 unnecessary fittings (and possible places to leak) in just one 15' run of hot and cold water lines. It had to take the workman longer to put in all those joints than run straight lines, which doesn't make sense if these units supposedly are built so fast as to make them cheaper.