I mainly have certs issued in England & Wales.
If yours is also issued in E &W, then look at the citation at the bottom, this is why we call them certificates - its certified to be a true copy of an entry in a register.
If its been issued by the local registrar (whether at the time of the marriage or at a later date) then the ones I have say
"Certified to be a true copy of an entry in a register in my custody" or words to that effect -in other words the original register, signed by bride, groom and witnesses at the time of marriage which is held at the registrars - under lock and key and the general public is not allowed to even peek at them.
If its been issued by the GRO, then the ones I have say
"Certified to be a true copy of an entry in the certified copy of a register of Marriages in the Registration District of [insert local registration district] "- in other words the local registrar made a copy of the the register and passed that along to the GRO - and they have issued the cert from the copy that is held at the GRO.
I understand that recently 'some' but by no means all, local registrars have the facility, like the GRO to issue a copy having scanned the entry in the original (signed on the day by the bride, groom, witnesses etc) register and printing it onto an offficial cert.
So its a minefield really, best advice is look at the 'handwriting' if the signatures look to be in the same hand as the info in the main body then its all been written by the same person who issued the cert. If each signature differs in style from both the writing in the main body and the other signatures
then its likely to be 'original' usually if its a parish marriage register entry or a local registrar who has scanned the original register - or a much later marriage than mine where they may have changed the procedure.
Boo