it's the microns - hair width - that tell you how soft it is.
but I'm not sure that's on every label.
here are the grades:
* < 17.5 - Ultrafine merino
* 17.6-18.5 - Superfine merino
* < 19.5 - Fine merino
* 19.6-20.5 - Fine medium merino
* 20.6-22.5 - Medium merino
* 22.6 < - Strong merino 
* < 24.5 - Fine
* 24.5–31.4 - Medium
* 31.5-35.4 - Fine crossbred
* 35.5 < - coarse crossbred
so I guess if it said on the label ultrafine or superfine that would be best.
but those are also the ones that pill most, I heard.