It is almost impossible to "find the value of a domain before registrering". There are numerous models of domain name valuation. Domain valuation is not exact science. Overture is a search engine which discloses statistics of what people type in the search window (roughly put) so you figure lots of people type in the domain name you are eyeing then bingo! More experienced forum members will tell you about overture numbers and valuation models of choice.
Start at www.overture.com and look for searchsuggestion tool or try this shortcut it may work for you http://inventory.overture.com/d/searchinventory/suggestion/