On February 5, 2018, NBC News reported that Julián Castro will "test 2020 run," visiting New Hampshire as the featured speaker at the February 16 New Hampshire Young Democrats 2018 Granite Slate Awards Dinner. The Federal Election Commission has made clear that travel for the purpose of determining whether an individual should become a candidate constitutes "testing the waters"--so all travel expenses must be paid for with candidate-permissible funds (i.e., $2700 limit per donor, no corporate/union funds). If Castro's not paying for this trip with candidate-permissible funds, he's seemingly violating federal law.
"I have every interest in running" in 2020 for president, Castro told NBC News. "Part of the process of figuring out whether I'm going to run is going to listen to folks and feel the temperature" of voters.