How exactly do you gather information about the frequencies and ranges of your opponent?

I dont own a hud therefore is very hard for me to gather information and there is a lot of variance at hu , i find myself playing mostly readless , one way i know to gather information is to go to showdown but i know no other way and even then the opponent might change his play , for example he may be passive oop with sdv hands but bet some on them on position or he might change his gameplay after a blind or two.