How to tell if plaster walls are insulated

Our home was built in Michigan in 1929. It's a cinder block basement w/
cement floor, and at the top of the basement wall we can see where wood
framing rests on the top of the cinderblock, so I'm assuming it's a wood
frame house on top of the cinderblock foundation. I see wood panels under
the aluminum siding outside (which is where chipmunks are going, but that's
a different story).

Our house is IMPOSSIBLE to heat, so I'm thinking we have no insulation. The
walls inside are plaster. When I look at the little cubby space behind the
wall in the 2nd floor (it's a sloped roof, and the walls of the room are
straight, so there's that little unfinished triangular space where you can
store stuff behind the wall along the length of the house), I DO see some
kind of old insulation in the rafters; it's black paper with all this
crumbly red nasty stuff behind it. It looks like the equivalent of the
fiberglass stuff you see today, just all crumbly and cruddy. I can't look
down the walls, though, because the slope goes out past the walls, so the
floor of the cubby space comes to a point where the sloping roof is going
down.

Is the only way to tell what's behind our plaster cutting holes and tearing
out whatever we come to until we see either insulation or the outside
surface of the wall? Might there be a better, less damaging way (like
taking one panel of aluminum siding off, removing that big wood siding
piece, etc., except that actually seems harder)?

I'd like to know if we can use that blown insulation stuff, but I think I
need to know what's behind there to know if it's possible.

Thanks for any tips, ideas, etc.