Armor, weapons and bikes will start to glow when you enhance them. The amount of glow depends on the amount of enhancements.
Armors: enhancing to 4 or higher. At 7+ the glow will get bigger and can shine through outfits as well, and even show in the club while wearing only lingerie (or less)
weapons: not sure where these start to glow
Bikes: at +1 the exhaust will burn, at +4 the headlights will glow, at +7 there will be a glow all aound the vehicle, and I heard it will smoke at +10