This would greatly depend on what you are doing? If you are looking for a refi, then the lender will send someone out to appraise the property. If you will need it for a personal reason (ie divorce) then the lawyers either require an appraisal or three differant CMAs from real estate brokers.
These are the only two ways of figuring out the value of the home. From there, you would subtract what is owed. Whatever remains is the equity.
Hope that helps,
Terrence Charest, e-Pro