Look at your brake lines where they connect to the master cylinder. Manual MC is very close to the firewall. Power brakes move the master farther away from the firewall and up, because the booster is angled. Also look at the firewall for marks that the booster was there. Booster uses all 4 bolts. Manual only uses the top two bolts and the lower two will still be there with nuts and lock washers. Look inside below the MC hole. If it had manual brakes, the master cyl would have leaked at some time and there would be evidence of fluid leakage.
Booster cars have a lower pedal height, even with the clutch. The brake light switch contacts a metal tab bolted to the top (manual) hole in the brake pedal and there should be evidence the lower hole on the pedal was being used. If power brake, there is no rubber bumper for the pedal because the booster controls pedal return.
If manual, the top hole is used, no metal tab for the brake switch, rubber bumper is used for pedal stop, there may be a return spring. Nothing in the lower pedal hole.
Power brake setup: still has rubber bumper because it was converted from manual brakes by the dealer.