SABuzzard,
I'm in the process of building up a rifle with the same basic specs as yours (10" SBR, AR platform in 300 BlkOut) so this one caught my eye. The pressure signs for full charge loads in ARs are essentially the same as they are for any other autolader. Micing the cased head is a good place to start, and primer appearance is always something to keep an eye on (even if they're not always one to be believed). Chronographing is another, since the capabilities of the 300 Blk Out are already pretty well documented. In other words, if you're getting substantially more velocity than pressure tested loads without other pressure signs, then the velocity itself IS a pressure sign. Condition of the primer pockets are another one to pay attention to when reloading those cases. Are the pockets still snug? Does it take a degree of pressure to seat the next primer, or does it drop in with very little noticable resistance? What I'm saying is, absent the standard of bolt lift "feel" common to bolt guns, you need to take a look at a broader array of pressure signs, and take them as a whole.
My bigger concern is with subsonic loads, as this is such a departure from most loading experiences. I suspect I'll be watching my chrono more than anything else with this one, but it should be an adventure! Looking forward to starting this one once all the ATF paperwork clears. Keep us posted on what you find!