There is no website that I know of other than what Realtor's use to compare homes by area and type. This is best accomplished by choosing a Realtor to give you a Market Analysis on your particular home. You may be daunted in asking for a Realtor's help if you are not ready to sell, but that sincerely, Lisa, is part of our job. We need to serve our client's best interests over and over until they are ready to make a move. The more willing the Realtor is to help you with this service, the more likely it will be that you will choose that Realtor when it is time to sell.
The equation I use to let someone know when it is time to sell a home is; what the client proposes to do with the money from the sale. In example, if you were to take money out of one investment to place it in another that will turn a higher rate of return, the answer becomes a little easier on the mind.
I hope you found this answer helpful.