There really isn't a good way to tell with out a guage. If you ran the saw when it had good compression, you can compare how the rope pulls now to how it used to pull. Also, if the saw is down on power and smokes more than it used to using the same mix ratio, you can guess compression is low. Without a guage you won't know for sure.
You can pick up cheap compression guage at the autoparts store or one of the online tool places for not much money. They are not the most accurate, but they will at least ball park it for you.