Actually you have a little more time than that. The sale has to be confirmed in court. This involves the bank’s attorney filing a motion to confirm the sale; you should get a copy of that. At the confirmation, a judge must sign a court order (a copy should be mailed to you) which will give you additional 30 day to move out. If nothing else is done, you have a legal right to remain in your property until the end of the 30 day period.
You'll know that the property was sold when the bank files a motion to confirm sale with the court. Or, when the bank sends you notice that it was sold. If you did not file an appearance in the court case the bank is not required to send you notice, but most of the time they do anyway. Also, you can call the bank's attorneys and they will probably tell you when it was sold, and if not, when the sale is scheduled to occur. As far as letting someone in to do an appraisal, make sure it is a representative of the bank, but although you own your home until possession has been transferred (see answer of other attorney) your mortgage documents probably say that the bank is allowed limited access to the property for valuation or protect its interest in the property.
The information in this answer is not intended as legal advice nor do I intend to create an attorney-client relationship with any reader simply by answering this question or contributing as a member of AVVO.
There is not real way to know if the property is sold if you do not attend the public auction. But you can call the foreclosure attorney and ask them the status. They are generally honest. Otherwise you will have to go the court house and pull the file and look for a "Motion to Confirm the sale". You may see it on line but I doubt it. If you go on line and see a Motion after the notice of sale then that is likely the Motion to confirm and you will know that the house was sold. Also you do not have to let them in. They normally just do a BPO and not a full appraisal so that call is surprising. It is true that you own the home until you don't. The bank has to sell the house and the sell has to be confirmed. In very limited circumstances you can get a sell vacated but otherwise the judges usually confirm the sheriff's sale 7-10 days after the sheriff's sale. At the time, your are no longer the owner and the new owner is the person that purchased your home at the sheriff's sale.