Look for FX compatibility, ED (Extended Definition), Nanocrystal Coated for flare resistance, and a gold ring. And just ask your dealer!

Also look for a price tag that says, "I'm expensive as hell," compared to the cheap version of the same focal length or range. If it costs like an L lens, it's most probably similar. It's going to be weather-sealed, have a wicked-fast focus motor, exotic glass...

Most pro lenses also have wider apertures, and CONSTANT f/2.8 apertures if they are wide to medium tele range zooms (i.e.; 14-24, 24-70, 70-200).

"FX" versus "DX" is not useful to distinguish "high end" from "low end", "entry level" or "budget/kit lens". For example, a Nikkor 10-24mm is sort of mid-grade build, while their 12-24mm is more pro-quality build... and both of those are DX lenses. But, that also doesn't mean squat when it comes to price versus performance. For example, at roughly half the price of those Nikkors, either Tokina "ATX Pro" 11-20mm f2.8 or 12-28mm f4 offer as good or better image quality, as good or better build quality and roughly equal general performance. Incidentally, Tokina uses the same "DX" and "FX" terminology as Nikon, and both the Tokina lenses mentioned are DX models.

It's similar with Canon... there are both high-end and low-end EF-S lenses... but there are no L-series EF-S and never will be. That's merely because one of Canon's criteria defining "L-series" is that they must be compatible with and usable on all EOS cameras past, present and future. That's not the case with EF-S lenses, which can only be used on APS-C format Canon DSLRs. Therefore, no EF-S will ever be "graced" with a red stripe or the L label.

Yet, there are some EF-S lenses that rival L-series for image quality and in most performance characteristics.... such as the EF-S 10-22mm, EF-S 17-55/2.8 and EF-S 15-85mm.

Things that might distinguish "high end" Nikkors:

- Larger apertures
- Non-variable aperture
- IF or "Internal Focusing" design
- APO, ED, UD or FL glass
- AF-S... i.e., a focus motor built into the lens
- SWM or "Silent Wave Motor" focus drive
- VR or "Vibration Reduction" image stabilization
- Gold ring
- Nano coatings

You'll generally find several of these factors on the higher end lenses.

AF-S lenses are typically newer models. There are high-end AF-D lenses, too (without built in focus motors), but those tend to be older models. Nikon has gradually been converting their lens line-up to AF-S.

Not all AF-S lenses are high end... there are also lots of entry-level models. So, by itself, AF-S only indicates it's probably a relatively recent model (however, the earliest AF-S were introduced almost 20 years ago).

And, of course, price is one more thing that might give you a clue!