Depending on how large/important your collection is, I would consider the following:
1. Go to the main auction houses and ask them for estimates and reserve prices (and commission rates)
2. Find one (or more) expert appraisers that can help you scour the market for references and comparables. Usually standardised valuations from places like IGL are aimed at replacement/insurance value, which has nothing to do with the price you may be able to get when selling
3. Talk to people like David (the owner of Diamonds by Lauren). They may or may not be interested in buying themselves, but they can point you in the direction of people who can and can also give you interesting pointers in terms of prices/values
4. Sites like the one that Peo is asking about on the first post of this thread also can help - however as David mentioned above there is no strong link between results at one specific auction and what a similar piece may do tomorrow.
Perhaps obvious, but worth mentioning that the 4 suggestions above are not mutually exclusive.