You can buy a dissolved oxygen meter, but they'll generally start in the US$200 to US$400 range, and a suitable one I believe runs closer to US$500-600. Not exactly an everyday piece of equipment for a measurement you're going to take once. There are cheaper test kits designed for other applications, but those will only get you in the approximate ball park, so I'd say that's not really going to be much use in this application.
The typical way to know if your fish are getting enough oxygen is to observe if your fish are continually gulping at the surface of the water. Oxygenation occurs any time oxygen-poor water comes in contact with the air at the surface. But there's no magic to an air stone that is going to "put oxygen" in your tank. Aerators and such are just another way to circulate water and to create surface agitation. Any filtration system (or even a simple circulation pump) that takes in water from below and releases it towards the surface should provide sufficient agitation in your tank to meet your fish's oxygen needs… unless you are overloading it.
Honestly, if you're overloading the tanks enough to worry about oxygenation, I think you will run into a lot of other problems first. Usually the number/size of fish an aquarium can support is determined by the nitrogen cycle (the ability to get rid of waste), not oxygen absorption. But if your fish are gulping at the surface for oxygen, you'll either have to increase the water circulation, or reduce the number or size of your fish. But buying an expensive piece of equipment to know the exact mg/L oxygen saturation isn't a typical diagnostic for the home aquarist.
