Surveys suggest a shocking number of homeowners -- as many as one in three -- don't know if they have a fixed-rate or adjustable-rate mortgage.

Nearly half with ARMs aren't sure when their loans will begin adjusting and three-quarters have no idea what their new interest rate or payments will be.

If you're uncertain about any of this, dig out your mortgage papers and check. A big, unexpected bill landing in your mailbox is no way to find out.

This is especially important if you live in a state like California or Nevada, where ARMs account for 40% of all outstanding mortgages, or have a subprime loan, which almost always has adjustable interest rates.

It's not unusual for monthly payments to increase by 30% or more when ARMs reset for the first time. A recent poll sponsored by the AFL-CIO put the average increase at $291.

If your payments are going up, click here for Interest.com's advice on how to cope with rising mortgage payments.