[Schroedingersat]

Probably the best and simplest indicator is to look for 8 or 9 speed on the back wheel, or a premium rear gearbox like a rohloff, or either a 3 or 5 speed sun gearbox like a sturmey archer or shimano 3 speed.

Brands will vary both in servicability between models and which hrands are available.

Avoid any integrated electric parts that the frame is builtaround (but a bolt on motor and battery is fine whether the motor is fitted to the wheel or pedals).