If someone has taken out a mortgage without your knowledge this is considered FRAUD. 

If you know the address of the property or the county that the property is located in, a local Title Company should be able to do a search of your name and find out what mortgages you have out at this time.  Or depending on the county you maybe able to access these records yourself on the counties online system if they have one available. 

If you do find a mortgage on record that you have not taken out you need to get in contact with a real estate attorney as soon as possible, this again is fraud.  A good real estate attorney should be able to help you sort things out.

A lender is not required to record a mortgage but they almost always do.  This is done at the registry of deeds or the land court in the county where the property is located.  It isn't difficult to run the title on your property and there is usually someone who works in the registry who is willing to get you started.  You probably don't need to go back further in time than the last mortgage you took out yourself to purchase or refinance the property as your lender would have run title then and certainly would have informed you if he found a lien senior to his own.

If you don't mind spending a little money contact a title company to do the search for you.  It isn't terribly expensive.

Title searches as Jill and Jann mentioned are great ways to see if you name appears on the title of property.  Usually, the title profile will list the lender and loan amount as well. 

A home loan will also appear on one's credit report.  Check your credit report as that will surely tell you if some has taken a loan out in your name.  You can obtain a credit report by asking for one at your bank or through a mortgage broker (you will pay a small fee) or you can run your run your own credit report online at www.annualcreditreport.com or through the major reporting agencies (Equifax, Experian and TransUnion). 

If your suspicion is verified, you need to file a police report (as it is a crime), contact the lender and perhaps sign up for a credit monitoring service through your bank (I do not know the quality of these services).

Let us know how you progress.  Good luck to you.