How to check the type of installed insulated glazing
Louis it is a really good question.  First if you have the space to put the cheaper insulation then you can simply buy more of that to get the same results.  If the insulation type is the same but different U value then it only matters what total you are getting.

How to verify this?  This has been answered on the site many times - a couple by me.  There isn't an exact way but you can get a rough estimate.  Remember you are also measuring the installation not just the insulation.  If not packed correctly and there are gaps then you will have the same poor performance.

How do you know what a contractor installs?  You watch them do it.  If the insulation is covered in walls or whatever then there isn't an exact way to tell that they did or if it is the right stuff.  You need to be there and take pictures and ask questions.  A good contractor won't care if you are doing this.