Hi joycejoy,
Welcome to the Mattress Forum! 
Just like with a traditional mattress, you should find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a sofabed mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase. I know this will be more difficult with a sofabed mattress, as they tend to use more basic materials and are limited in the overall amount of material that may be used.
I would definitely recommend trying out anything before making a purchase. Post #2 here also includes a few good sources for sofabeds.
Phoenix