In marriage records, you may often find a note that the parents had died. Depending on the time and place, this was not necessarily the truth. Saying the parents were dead was an easy way to avoid having to prove parental permission.
If it is a civil registration marriage record (after 1811 in most places), you may find proof of death in the marriage supplements. The law stipulated that a person under the age of 30 had to provide parental permission or proof of death of the parents, or even grandparents if both parents had died. The documents to prove the death became part of the marriage supplements. If those includes an extract of the death or burial record, you can be pretty sure the parent was actually dead. If the proof consisted of witness statements, it is less certain. If the person married in the same town where the parents had lived, it is reasonable to assume the parent had in fact died, though perhaps not exactly at the moment the witnesses recalled years later. It would be hard to fake the death of a parent in a small town. If the parent lived in a faraway place, it may have been convenient to have witnesses swear that they had known the bride or groom for many years and always heard him tell the parents were dead or something to that effect.
If a person married in a large town before the introduction of the civil registration, it was much easier to just claim the parents had died, especially if the parents lived far away from the place of marriage. I always take those declarations with a grain of salt.