how can I check the hardware compression ratio.
Pick a medium of type FULL (that is, one that was written until full), get the amount of data that made it to the medium from the Usage tab in the GUI, then divide by native capacity of the medium?
Tape : HP MSL 4048
That's not a tape drive technology, it's a library. Native capacities of media types compatible with drives typically found in a MSL 4048:
So for example, if your full LTO3 medium shows up with 771460MB, the compression ration of this particular data blob was approx 1.93:1.
You could script it using:
omnimm -media_info MEDIUM -detail
That will give you a Total blocks figure that you can use.