How to detect if a certain section exists ?

I read that the error flag is set if the section doesn't exist but it doesn't seem to work for me while I'm using:

SectionGetFlags ${secid} $R0
IfErrors FinishPage

and yet it continues without going to FinishPage even when the section doesn't exist.

If the section doesn't exist, ${secid} probably doesn't exist as well. If it doesn't exist, you're checking the first section. The first section has the id of zero, which is exactly what ${secid} is trasnlated to. It's translated to zero because it contains no digits.

You probably want to use !ifdef secid.