Hi
I think there were laws / guidelines, but from what I've seen, you have to not asume anything!
One pair of my predecessors registered and gave great detail as a married couple for two babies and I never found a marriage. Then I looked a good few years later, and found that they married when they had two children, the first now five years old : Both registered as if the parents were married.
The icing on the cake came recently when a Rootschatter found a baptism for a child I did not know about, born a good few years earlier stil - registered under the father's surname with the mother's maiden and in fact current name a if they were married - but all the other details were missing!
I think you just have to look at all the clues going, and in both parents' names if there's a problem.
If you cn get a baptism register though, as Heywood said, the clergy's cryptic commentscan say a bundle! A common comment up here near the bordewr is, "Parents supposedly married earlier in Scotland"! (Variations such as Gretna Green", "Married they say", ---etc!
Have fun!
Emms
