Usually one would use a DO Meter, but they can get expensive. A few people on the forum have different brands. IIRC Milwaukee is one of the least expensive brands. The lower cost unit's lack in features like altitude, temperature and salinity compensation. They may also be less accurate.
Another method is Titration. Not too expensive, but needs to be done quickly and carefully. May have to practice a bit first.
DO Titration LaMotte is one of the leading brands. If going this rout, you may want to browse around and see if there are any other water parameters you would like to consider. May get a better price for an Aquaculture kit.
