You'll get about as many ideas here as you will get responders......
I'd go with what GG said but.......first I'd try this:
Make a plumb bob. Mine is a very long piece of string (celing to floor length) , a partial neck sized case (your 300 RUM would be perfect) and a bullet.
That is, if you don't have a plumb bob readily available.
Stick the string through the flash hole and out the mouth a ways. Tie a large knot in it so it won't pull back through the flash hole. Pull the knot to the back to the case head and set a bullet in a little ways.
Find the longest open span in the house. Like from the bathroom down the hall and into the garage, or what ever. Hang the plumb bob from ceiling to floor. While it stops swinging set up a table at the other end of the open area.
Mount the rifle on the table level. Use a cleaning vise and a bubble out of a cheapie lever for on top of any flat spot on the action. Top of rear mount, magazine rails etc.
level the rifle precisely, if that is possible with a bubble level.
Then center the cross hair on the plumb bob string. Reverifying level constantly.
Then turn the scope until things are parallel. You'll know then what you have.
Additionally once setup solid and lever. crank up the elevation and see if the + reticle goes straight up.
GG's method is way more nifty but this way is way more fun. Plus anyone else in the house get to gen up some interesting comments.;)
But hey, it works.....
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!