Some manufacturers recommended flipping a pillow-top mattress to extend its useful life, while others call their mattresses one-sided, causing confusion over the right thing to do. The answer really depends upon the mattress itself -- look at the other side of the mattress if you no longer have the original information from the manufacturer or retailer.
The base portion of of a pillow-top mattress looks just like a standard, old-fashioned mattress. The "pillow-top" portion adds an extra cushy layer of support several inches thick, and it does resemble a large pillow or featherbed on top of the mattress, in some cases. On many pillow-top mattresses, this pillow-style cushioning is only on one side of the mattress -- the top.
How to Tell if Your Mattress Flips
Remove the bedding and note the construction of the top of the mattress, especially where it meets the edges of the mattress. Lift a corner of the mattress and look at the bottom -- if it looks billowy and fluffy like the top, it can be flipped. If the bottom looks different in any way -- flat or with a rubbery gripping texture and a surface different from the top -- it is not meant to be flipped. While there's nothing preventing you from flipping your one-sided mattress and using it wrong-side-up, this results in premature mattress wear. Sleeping on the wrong side of the mattress is also not likely to be comfortable, as the inner structure of the mattress is layered for optimal comfort and support, designed to support the body in a particular way from one side only.