When a birth, marriage or death is registered, the local registrar retains a copy of the details and then forwards them on to the General Register Office (GRO). The GRO index these - i.e. they compile an alphabetical list of the names of the individuals concerned. In the early days of civil registration, these indices contained only the names (plus the GRO's own reference). In later times, they contain slightly more (mothers' maiden names for births from 1911, age for deaths from 1865 etc).
There is no direct way to find out the information on a certificate without buying it, but if you already know (or think you know) some of it, you can reduce the risk of getting the wrong one by asking the GRO to perform a check for you before they issue the certificate. Doing this is a bit fiddly, and takes longer than a normal order but it is worthwhile if there are a number of possibilities. To access this facility, you need to make sure you check the "no" box in the section of the GRO certificate order form which asks if you have the GRO reference. On the next screen you will then be able to specify certain criteria (such as parents names for births) to narrow down the search. A certificate will only be issued if a match is found; otherwise you will not be charged.
There are other ways, too, to help identify the correct entry, using census data, parish registers and, in the case of some registration districts, local registrars indices which may contain more information that the GRO copy.
If you would like to post some examples of your elusive ancestors, we can give you more specific advice, based on the resources available for different dates and localities.
Caroline