It's a tough question - if you buy cheap unbranded parts you can usually tell straight away that the plating and polishing isn't such good quality. What's harder to judge is the quality of the metal itself, and whether it's been hardened to make it fit for purpose - especially important with things like tremolo systems.
Having said that, I don't have very much experience of the cheaper stuff. I usually buy branded parts - Gotoh (or TonePros), Schaller, some Fender, Callaham. Are they actually "worth" 2, 3, 4 times the price? Maybe not, but they're certainly very good.
I did try one of the cheaper Wilkinson tremolos (as used on Vintage guitars) and it's actually very well made, although they've clearly saved money on the screws and springs. But in that case, even though the parts are made cheaply, Trev Wilkinson is prepared to put his name on them so you assume there's a degree of quality control.
Personally I'd steer clear of the really cheap, totally unbranded stuff because - I suspect - you get what you pay for.