1. Colour that was measured, not admired
The single biggest separator. An amateurish job has colour that drifted: skin tones gone brick, brand blue gone purple, greys gone green. A professional job was printed against a target, measured with an instrument, and corrected. The public machinery for this is ISO 12647-2 (the offset process-control standard defining what the measured sheet should hit) and the International Color Consortium's profile system, the vendor-neutral format (ICC profiles, standardised as ISO 15076-1) that lets a designer's file, a proofing device and a press all describe colour the same way. A shop that can talk about its press profiles and show a measured control strip is telling you quality is a process; a shop that says "our operator has a great eye" is telling you it is luck (the G7/ISO relationship is unpacked in our colour systems piece).
2. Register
The four process colours print as separate passes, and they must land on each other precisely; look at fine black text over a coloured background, or any hard colour edge, with a loupe or a phone macro lens. Halos, colour fringes or doubled edges mean the job is out of register. The registration marks in the sheet margin exist so the press crew can check this continuously; it is the oldest quality discipline in the trade (and the mark this masthead wears in its own nameplate).
3. The same brand colour everywhere
Brand damage rarely comes from one bad job; it comes from the letterhead, the carton and the banner each being a different blue. Consistency across suppliers and processes is a specification problem: define the colour once (a spot reference, or process builds per stock type), state which stock and process each application uses, and require measured proof against that definition rather than "match the last one". Where the colour is out-of-gamut for CMYK, decide deliberately between a spot ink and an extended-gamut build (the decision framework).
4. Proofing that means something
A proof is a contract about appearance. A calibrated contract proof (or a press check for critical work) is the professional instrument; an office laser print or a phone-screen PDF approval is not a colour reference, and a shop that lets you approve colour from one is quietly moving the risk to you. Agree what the proof is, what it binds, and what happens if the run does not match it; that conversation is part of what a dearer quote buys (how pricing works).
5. Files that did not fight the press
A large share of "unprofessional" output was never the printer's doing: RGB images nobody converted, text set in rich black, missing bleed, 72dpi logos scaled up. The fix costs nothing but discipline and is covered end-to-end in our artwork prep guide; the short version is that a print-ready PDF/X file with correct bleed and resolution removes most of the ways a job can go wrong before a plate is ever made.
6. Finishing and handling
Cracked folds across heavy solids (score before folding on heavy stock), trims that clip type because nothing respected the safe zone, marking and set-off from mishandled stacks: finishing is where a technically fine print run can still ship looking cheap. The tell of a professional operation is boring consistency: square trims, clean folds, no marking, copy after copy.
What to ask a printer, in one breath
Which standard or calibration your colour is controlled to; what proof I approve and what it binds; what stock this quote assumes; which finishes are in-house. Four questions, and each has a checkable answer. Professional shops enjoy being asked; the other kind changes the subject.