Q about discrimination and "sincerely held religious belief"

[DetlefK]

How could one person possibly prove that acting in a certain manner is based on a "sincerely held religious belief"?

By swearing that it is so?

By pointing out the dogma in question in a sacred text?
But would this then qualify as "sincerely held religious belief" (e.g. Jehovah is the only god) or merely as "religious belief" (e.g. not working on Sundays, no bacon)?
And it even gets trickier with religious teachings that were added after the founding-phase (e.g. murder is okay if you do it on a crusade) or that were created from cultural traditions made up at later times (e.g. Christmas).