The lab I run on my patients is a fasting lactulose breath test which measures the gases (methane and hydrogen) released by the bacterial overgrowth. (mindbodygreen.com)
The BEST test you can take is a 3 hour lactulose breath test that you can take in the comfort of your home and send back to the lab in the mail. (wellnessmama.com)
In support of this theory, one study found that many of the subjects who tested positive for lactose intolerance using the hydrogen breath test also tested positive on lactulose breath testing. (verywellhealth.com)