Hi luv4sale,
The first place to start your research is post #1 here which has all the basic information, steps, and guidelines you will find most useful.
Besides testing for PPP (Posture and alignment, Pressure relief, and Personal preferences) one of the most important parts of a successful mattress purchase is making sure you know the specifics of what is in your mattress. A mattress is only as good as its construction and the materials inside it and without knowing the details of all the layers and components in a mattress (in particular the thickness of the layers and the density of any memory foam and polyfoam in the mattress) there is no way to assess its quality or durability or make meaningful comparisons with other mattresses in terms of quality or value.
Warranties are also not an effective way to choose between mattresses and are often more about marketing and protecting the manufacturer than they are about protecting a consumer (see post #174 here about warranties).
In general King Koil (Comfort solutions) would be in the group of “major manufacturers” I normally suggest avoiding in the guidelines that are linked in the “read first” post unless you are able to find out all the more detailed information you need about the mattress to make an informed decision and it has good quality/value compared to other similar mattresses in its budget range.
Phoenix