The best thing is to be aware about your feeling when you get up in the morning. If you feel one or more of the following points, it is worth considering to look for a better mattress, viz :
You wake up feeling tired
You have been tossing and turning during the night
Your mattress has sag more than 1 from its initial thickness level
Your mattress has lumps
You hear lot of cracking sound
You are having signs of allergies