So what do you guys use to gauge near max pressures
in an AR? Other than the obvious flattened or blown primers?
Thanks!

Thanks Troy, I didn't think there was much to work with there.
The question came from a bolt-gunner looking to safely work
up some loads for his new AR. Told him to go by the book on
his reloads...

Any good loading manuel should have a list of what to look for along with a description and illustrations.

I consider a chronograph an indispensible adjunct for safe reloading practice.  Simply put, if you're getting higher velocities than the manual indicates, you are probably running higher pressures.  It's a safe bet that the people creating the data, usually with an expensive custom pressure barrel, will safely get higher velocities at a given pressure than will us end users.  There are no free lunches in internal ballistics.

Used in conjunction with standard methodology (primer appearance, reseating pressure, pressure ring expansion, etc.) a $70 investment will help keep you safe.  Depending on chamber finish, headspace, brass quality, and such you may find no outward signs of excess pressure save unrealistic velocities.  I have had this happen numerous times.  

A chronograph is especially important if you're using non-canister grade powders.

Sam

Another indicator when using a chronograph is velocity increase differences using the same components with incremental increases in powder charge. Usually and I stress usually when you hit the high pressure mark you will see a marked decrease in velocity spread instead of an increase.

For example:
Load 1 to load 2 show an increase of 40 fps, load3 shows 30 fps and load 4 shows 20 or a decrease from load 3 then load 3 is probably a max or near max load.

This should always be used in conjunction with other visual and audible signs. Once again it only applies when using the same components fired the same basic time of day or near identical atmospheric conditions.

Common sense always applies.