Use pictures of Sindy as a guide, you can buy the book History Of Sindy or use the images on this site under Sindy Dolls as a reference. Another good website is www.themagictoybox.co.uk
Your Sindy will be worth more or less depending on it's age and condition.
For Pedigree Sindy you can often determine the approximate age of the doll by looking at the markings on her head or her back.

1963-1965 = Made in England

1966 = Made In Hong Kong

1974-1976 = 033050X

1977-1980 = 2 GEN 1077 and/or 033055X

1981-1982 = 033055X

1983-1985 = Sindy 033055X

CHECK THE CONDIDITON OF YOUR DOLL LOOK FOR THESE THINGS.

Are there hair plugs missing from the head?
Are the eyelashes thick and full or are some missing? (Some later Sindy's don't have any eyelashes)
Check the paintwork on the face is okay.
Are there chew marks or pen marks on the dolls face and body?
Do the knees bend and click properly?
If the doll is "active bodied" check all her joints work, ankles, knees, arms etc.

BOXED DOLLS

Sindy's in boxes are usually worth more than they are loose. The dolls box is important. The better the condition of the box the more it is worth.

TERMS

Some of these terms are used to describe dolls on ebay.

NFRB = Never Removed From Box
MIB = Mint In Box
OOAK = One Of A Kind
MIP = Mint In Packet
HTF = Hard To Find
VHTF = Very Hard To Find
BNIB = Brand New In Box

Once you have done all this, you can start posting your doll to fashion doll groups. Dolls are only worth what a collector is willing to pay. If you want to sell your doll, become a seller on ebay. You can set a high "reserve price" this way if the bids are not high enough you don't have to sell the doll.
Also keep an eye on these auctions, see what other sellers dolls are going for. This is the best guide as to what you can expect to sell or insure your Sindy for. If you are interested in getting opinions of collectors to regularly visit this website you can post details of your Sindy's on the Identifying & evaluating forum.