TMAO has been in the headlines recently (including my favorite breathless overexaggeration, "THIS blood test will predict how long YOU will live") as a probably-useful predictor of cardiovascular events.
As far as I can tell, the only company that can do TMAO testing is Cleveland HeartLab. They can send a phlebotomist to you (at a cost of $100-$300), or you can arrange your own blood draw for $35. The test itself is $90, and also includes HbA1c, insulin, adiponectin, and uric acid. It looks like the best way to order is to call them: 866-358-9828.
They provide a sheet with information for your practitioner, including diagnosis codes to help with justifying "medical necessity (things like type 2 diabetes, hypercholesterolemia, hypertension, and metabolic syndrome).
I have a suspicion that the news interest in TMAO may be an example of a submarine PR-generated story by Cleveland HeartLab and/or Proctor & Gamble (who is working on products for managing TMAO's effects). That said, I'd personally toss it in my test regimen if money was no object.
My notes on TMAO
Chris Masterjohn's arguments that dietary choline sources like eggs and liver don't actually increase TMAO.
Chris's additional arguments that carnitine from red meat is probably not the culprit that most articles about TMAO imply, and that many forms of seafood are probably worse.
Low-dose aspirin may decrease both TMAO levels and its effect on clotting, at least with supplemental choline.
Plant sources of choline might avoid raising TMAO, but I think that might only be because of indole-3-carbinol content that appears to inhibit the enzyme that produces TMAO rather than something inherently bad about animal products.