After playing around a bit, i found that if i'm inside a sidebar, the $INFO['id'] differs from $ID. The former one seems to always contain the ID of the shown page in main, whereas the latter one holds the "currently processed" page. I gess sidebars are handlet by template, which renders them prior bevor content.
So IF this is correct, it could be decided by simply:
if ($ID != $INFO['id']) {
// WOW, i found myself inside a sidebar
}