Answer first
A fake 55mm Big Pilot copies the IWC Big Pilot's Watch 55. The real watch has a 55 mm case. Many replicas do not. Photos hide scale. Measure the case before you judge the dial.
Start with the case size
Put a caliper on the case at 3 and 9 o'clock. Do not include the crown. A real Big Pilot's Watch 55 reads about 55.0 mm. A 46 mm Big Pilot replica reads about 46 mm. A 50 mm copy is not a 55 mm watch. The difference is 5 mm to 9 mm. That gap changes the wrist fit and the dial proportion.
Check the lug width. Check the case height. IWC does not list every dimension on its site. If a seller cannot give a caliper photo, treat the size claim as unproven.
Check the movement type
The Big Pilot's Watch 55 uses a hand-wound movement. A replica may use a quartz movement or a low-grade automatic. Ask for a photo of the caseback removed. Look for a battery. Look for a balance wheel and bridges. A hand-wound watch has no rotor. A quartz watch has a battery holder. A no-photo seller leaves the movement unverified.
Read the dial and hands
Compare the dial to official photos of the exact reference. Check the font of the numerals. Check the print weight. Check the hand length. Check the subdial or date layout if the model has one. A replica often uses a dial from a different Big Pilot model. That mix is a sign of a copy.
Inspect the caseback and crown
Look at the engraving depth. Look at the reference number. Look at the limited-edition number. The numbers should match the model. The crown should sit centered. The crown should turn without grit. A loose crown or a shallow engraving is a defect. It is not proof of a fake by itself, but it supports the case.
Weigh the watch and check the strap
Weigh the watch on a gram scale. A 55 mm steel case has mass. A light watch with a 55 mm dial is suspect. Check the strap width at the lugs. Check the buckle mark. Check the stitch count. A strap that is too narrow for the lugs points to a generic replica build.
What a photo cannot show
A photo cannot show case feel, crown action, or movement sound. A photo can show dial print, hand shape, and caseback text. Use photos for details. Use a caliper and a scale for size and mass. If the seller blocks both, stop.