A bit like getting a new gas boiler fitted..... for an important job, check with the trade body. There's a Society of Wedding and Portrait Photographers and there's the British Institute of Professional Photographers. If a photographer is good enough, he/she should join these groups (if approved). If not, still join them and learn!
The Society of Wedding and Portrait Photographers (SWPP) is a puzzle, though..... I know one superb
wedding photographer in Dorset and she doesn't appear to belong to any group, while some others on the SWPP list are more like 'product photographers' and they do belong. Odd.
The British Institute of Professional Photographers (BIPP) is a little clearer ...... they have standards that must be attained before registration.
If anybody asks you to recommend a good wedding photographer..... point them to the BIPP (or at least the SWPP).
For budding professional photographers - like one or two in this forum - I'd say it's best to get registered/accredited. If you can't do that, keep trying.