Hi Jewels,
There are really two things that are important with edge support … just like with assessing a mattress in general. The first is the things you can feel. This includes how it feels when you sleep on the outside edge of the mattress and whether there is any feeling of roll off, how easy it is to get in and out of bed when you compress the edge of the mattress, and on whether it supports you well enough if you sit on the edge of the mattress (if you do because a mattress isn’t really designed for sitting).
The second part of edge support is what you can’t feel which is the durability of the materials or components that provide the edge support. There is more about this in post #2 here. In general the border rod is used as an attachment point for the coils in some types of mattresses but they don’t really provide enough edge support by themselves and would need various methods such as spring clips that attach to the top and bottom border rod or some of the other edge support methods mentioned in the link.
In general … if the edge support comes from a foam surround and you sit on the edge of your mattress on occasion I would tend to use the same general guidelines as for the quality of the materials themselves (1.5 lbs in a 2 sided mattress and 1.8 lbs or higher in a one sided mattress) as a bare minimum but if you sit on the edge of your mattress on a regular basis higher density yet would be better.
Edge support systems that rely on steel are usually more durable than edge support systems that rely on foam.
Phoenix