A simple checkup is a private matter and you should schedule it for the evening or weekend. The employer continues to pay you during sick days so don't be too generous with your sick days.
There are however some more rules that could possibly apply: (Arztbesuch während der Arbeitszeit)
If the timing of the examination is critical (e.g. blood test which has to be done in the morning), the employer has to let you go
Same if your doctor only is open during your work hours.
preventive checkups have to be done on your own time
So from a legal perspective, there are some cases in which your employer has to let you go (while paying you 'bezahlte Freistellung'), but you have to exactly communicate your absence (in advance!) and not call in sick, which would be a lie and could lead to an Abmahnung (or termination in case of repetition).
This also applies only if you have a fixed (full-time!) working schedule. If you are working part-time or on a flexible time schedule, the employer can ask you to go the doctor outside of your working hours.
So as a result:
Do NOT call in sick, but talk to your employer before you schedule your appointments.