The short answer

A fake IW371736 gives itself away in four places: the caseback engraving and reference stamp, the subdial layout, the day-date window at 3 o'clock, and the movement you can see or hear. This reference belongs to IWC's 3717 Pilot's Chronograph generation, roughly a 42 mm steel case running the 79320 caliber, which is a Valjoux 7750 base. That layout matters, because the 7750 family places its counters at 12, 6 and 9 and puts the day and date together at 3. Most copies cannot fake that geometry, and the ones that try usually do it badly. Start at the back of the watch, then work forward.

Check the caseback first

Most examples of this generation closed the back with solid steel rather than sapphire, which means the movement is hidden and the engraving does all the work. Genuine engraving is cut to an even depth with clean edges. The reference number, the case number and the maker's name sit where they should, and the lettering does not blur or thin out at the ends of a stroke. On copies the engraving often looks stamped and shallow, or laser marked with a grey, washed-out edge. Numbers get swapped, spacing drifts, and the font weight is off. If a variant you are looking at has a sapphire back, skip ahead to the movement section instead.

The dial is where most copies stumble

Subdial spacing and function

Real 7750-based dials have widely spaced counters because the movement underneath is large. Copies built on smaller or quartz movements squeeze the counters toward the center, and the date window ends up too close to them. Run the chronograph. The counter at 12 should sweep in one-minute jumps, the counter at 6 should track hours, and the small seconds at 9 should tick smoothly. Fake subdials that never move, or that spin when the chronograph is stopped, are the giveaway. Some cheap builds do not even cut the dial: the counters are printed and painted on.

The day-date window at 3

Look at the aperture. It should be cut square to the dial, framed cleanly, and the wheels behind it should sit level with the printing sharp. Copies show crooked windows, day discs that do not center, mismatched fonts between the day and the date, and a date that flips halfway through the night instead of snapping at midnight.

Print, hands and lume

Dial text is crisp and sits on the dial, not floating above it in thick paint. Hands are the right length, reaching their markers without overshooting or falling short, and the lume is filled level with the metal. Uneven lume, a short hour hand, or a seconds hand that wobbles as it travels are common on fakes. The crystal should be sapphire with a faint blue sheen from the coating. A strong purple tint, or glass that scratches with a fingernail, tells you what you need to know.

Crown, pushers and case finish

The crown screws down and should turn with a smooth, oiled feel. Pushers have a short, firm throw and return cleanly. Copies often have gritty threads, wobbly pushers, or a crown that sits proud of the case. On the case itself, brushing should be fine and consistent, with polished surfaces appearing only in the same places on every example. Look at the lugs from the side: sharp, uneven bevels and a case that feels light in the hand are typical of copies. A 42 mm steel watch on leather should feel like it has some mass to it.

Movement and feel

Set the watch down in a quiet room and listen. A 7750-based automatic has a distinct rotor wobble when you move your wrist, and the winding feels mechanical rather than thin. If the watch is quartz, the seconds hand will tick in steps and you can stop looking. If it is a hand-wound copy dressed up as an automatic, the rotor will spin freely with no resistance and no winding feel at all.

Box, papers and price

Paperwork only helps if the numbers match. The reference on the warranty card should read the same as the caseback, the serial should line up, and the card should not look freshly printed on a home printer. Pricing is the fastest filter of all. Secondhand examples of this generation generally trade in the low four figures to mid five figures in US dollars depending on condition, dial variant and whether the box and papers are present. A 42 mm steel IWC chronograph offered at a few hundred dollars is not a bargain, and nobody is selling one that cheap unless something is wrong.

Ten-second checklist

• Caseback engraving even, deep and correctly spelled, with matching numbers
• Counters at 12, 6 and 9, spaced wide, and all of them actually working
• Day-date aperture square, printing sharp, flipping at midnight
• Hands the right length, lume level, crystal with a faint blue sheen
• Crown screws down smoothly, pushers firm, case has real weight
• Price in line with the used market, not a fraction of it

Work through those in order and you will catch almost every bad example before money changes hands. When two or three of them fail at once, stop looking and walk away.