You could see what's in it at depth if you wanted to. When my pony was back-mounted, I had a pressure gauge on a long-ish hose clipped with a bolt snap to my left D-ring. When I switched to slinging it, I have the pressure gauge on a 6-inch hose tied in a typical way that it's kept where I can always see the pressure. Some people use a little button gauge.
It's good to be able to see the pressure of your pony/stage bottle in case it's leaking, since it is your emergency back up.
The only way I could see my pony bottle gauge at depth is to remove my entire rig.
I use a button gauge on my pony bottle which is mounted to my main tank. I check the pressure before the dive when I turn the pony tank valve on, and in I go. The odds of it developing a leak that drains a considerable amount of gas out of it (aside from a free flow from the secondary regulator that is tied just below my chin which I would notice) is infinitely small.