If the bike pedals are single sided you can tell which is left and right. Double sided pedals are marked - either on the flat end of the axle or beside the 15mm flats for screwing them on. Modern pedals are always marked 'R' and 'L'. Older French or Italian pedals may be marked 'D' and 'G' or 'D' and 'S'. The material of the pedal axle is usually harder than the material of the crank so take care as it's easy to damage the soft thread of the crank.
Grease the bike pedals
Coat the threads with anti-seize assembly grease to make them easier to remove when they have to come off. Anti-seize forms a chemical barrier between the two metals to prevent bi-metallic corrosion.