The DGA has long been the most reliable and accurate method for determining the internal health of oil-filled transformers. The analysis requires drawing an oil sample from the unit, extracting the gasses from the sample, and analyzing the gasses through gas chromatography. The results of the chromatography tell the lab about different problems that may be arising inside the transformer. Some modern transformers have gauges on them that read levels of dissolved gasses in the oil in real time. But usually, the oil must be drawn and sent to a lab.
