DO is measured in units of mg/l (or ppm, interchangeable units, 1:1) OR as a percentage value. The amount of O2 that can be dissolved in water is dependent on the temperature of the sample, so frequently a % is used to report the DO value.
There are test kits and meters for testing DO, as stated by others. The most important thing with DO accuracy is sample handling, as DO is a constant moving target. Leave a sample in a sealed jar, and bacteria will consume O2, or planktonic algae will produce O2. This is why, as stated by
S
skjl47
earlier, samples need to be "fixed" in the field if they will be tested at a later time. For this same reason, I prefer a direct reading meter for surface water sampling. This is more complicated if you are collecting samples at depth, for obvious reason... Here is a link to the LaMotte brand kit, and in the link there is a very detailed instructions manual that give you a good bit of information with recommendations for techniques and some of the science behind the methodology.
http://www.lamotte.com/en/aquarium-fish-farming/individual-test-kits/5860-01.html
As for DO meters, there are two primary types, galvanic and optical. Both are calibrated in the same way, by setting 100% concentration in a closed vessel, measuring the 02 level in moist air over water or a wet sponge.
Galvanic membrane probes have been around a lot longer, they have a membrane and electrolyte solution that must be replaced, which is a drawback. Also, galvanic sensors are consumptive, meaning that that consume oxygen from a sample as they measure the concentration. This has obvious drawbacks if you are working in a small sample, or if the membrane tip is resting on the bottom of a container, as it will consume the oxygen in the local area in which it is reading, and give a false low reading.
Here is a link to one particular galvanic DO probe that I use regularly:
http://www.wtw.de/us/produkte/labor/dissolved-oxygen/portable-meters.html
Optical DO probes use luminescent technology to measure O2 concentration. Essentially, an LED light replaces the membrane and electrolyte of the galvanic meters. These units require less maintenance, and the primary advantage is that they do not consume the O2 from the sample as they measure. There are many brands of these out there, and from my experience, they all perform similarly.
Here is a link to the one that I am currently using:
http://www.vernier.com/products/sensors/dissolved-oxygen-probes/
All that being said, the LAST thing that I would suggest spending money on in terms of water quality measurement is anything to do with DO! If you have ANY agitation of the surface of the water, or ANY form of aeration (which also leads to surface agitation...) then you will find that your DO values in your home aquarium will be consistently at or near 100%. Measuring DO in natural water is much more important, whether there has been a chemical contamination that limits DO, or simply an excess of respiration from bacterial bloom, of if you are investigating benthic zones of lakes or ponds where decomposition exceeds photosynthesis. Worrying about the level of oxygen available to your fish makes about as much sense as worrying about the level of oxygen available to YOU! Yea, if you close yourself in a box (or seal the surface of an aquarium by excluding all surface agitation), you will have issues. If you want to spend money on testing equipment, we can talk about pH meters, ion selective electrodes for ammonium and nitrate, conductivity meters, or even simply higher quality test kits than the standard API "master" kit.
While typing this, I have been simultaneously monitoring the pH in my angel tank, which I am in the process of boosting from the horrific level of 3.8 through the addition of calcite... it is up to about 4.25 at the moment, took a pretty big jump initially, but is gradually climbing now as the calcite slowly dissolves. I wish I had thought to plug in the conductivity meter to record that change a the same time...
