Hi Jacki
Well you have to be a bit careful as you are moving in to the realm of living people and searching them or enquiries about living people have to be protected but there are things you can source from the public domain which will help you.
You need to start by finding or possibly buying birth marriage and death certificates. These may be 'findable' in family documents, wills, bibles etc. If they cannot be found then you have to buy them.
On a birth cert you will get a great deal of info but of course you need Dorothy's maiden name for that so you may have to begin by sourcing a copy of her marriage cert - from that you will get her address at the time of her marriage, her name before she married and possibly - if entered her fathers name and occupation. The names of the witnesses at the marriage may also give you a clue - a sister or brother perhaps and so on. That helps you part way to finding her birth name - from there you may get a location so that you can source a copy or copies of birth certs for her.
It may be that from her death cert you will find a name of the informant at the time of registration of the death?
This can all become very expensive and that is why this forum is so good as there are loads of chatters out here who will help you along the way - perseverance and patients - is that how you spell that - is essential and eventually a healthy bank balance is needed as well!!!
PM me for more info if you would like.
regards and hope this helps a little
Nigel