The best place is to monitor Autotrader or similar, these will give you an indication as to what prices cars actually sell at. Looking in Parker's guide or similar will give you an idea but bear in mind that these people sell magazines, not cars so they need to tell customers what they want to hear in order to sell mag's.
If you need a quick sale, it could be worth looking into auction but if you want proper money for the car, this isn't the best place to achieve it.
Make, model, mileage, colour? What is it you're selling, I'll have a look and give you an idea if you want...
JJ