Flintkote under siding

[NWhops]

I need to know what this Flintkote stuff is under my siding. I can see it from the inside of my attic and garage where the stud cavity is exposed. It is under 10" lap cedar siding on the outside. House dates to 1955. It looks almost like particle board.

How can I know for sure if it is asbestos?

Thank you!

[HayZee518]

that stuff is the old style homosote. the outside skin is tarpaper. it is compressed cardboard. no asbestos fibers inside.
it is basically insulation board. compressed cellulose.