Monthly check:
-wheel pressure
-chain tension
After a year:
1) barrel adjuster
2) direction control system (cable: you'll see metal powder on the direction unit when wearing out) (banana), (roller x2: there are 2 column shaped parts in black colour inside the steering unit, guiding the wire. These may be worn by now), (does COGY go straight when free from steering?)
3) chain tensioner
4) caster wheels (are they moving smoothly?)
5) welding. especially the bottom of the backrest is sometimes reported to have cracked after a few years, especially with users with larger body build.
6) wheel thread