Like Petrol, I too generally load plus P cartridges in Nickled casings, but a more visible sign is the projectile used. A wad cutter or Round-nosed Boolit indicates standard velocity or below, while my +P loads are hollow-points, jacketed or cast. Makes it easy at a glance to tell which is which.
My Model 38 has a cylinder gap of .10 now, thanks to a heavy diet of plus p loads, back in the day when it was my only back-up/off-duty weapon. The admonition to avoid heavy loads in aluminum framed guns is well heeded.