On April 24, 2019, the Washington Post wrote an article titled "Trump suggests Mueller ‘checked my taxes,’ despite no mention of them in report":
President Trump suggested Wednesday that special counsel Robert S. Mueller III and his team examined documents related to Trump’s personal finances as part of their Russia probe, despite the fact that Mueller’s report made no mention of doing so.
Is there any evidence to corroborate Trump's statement that Mueller's team 'checked' his taxes?
Not that we know of with certainty... yet...
It is known that Robert Mueller's office referred several matters to other US Attorney's offices for further investigation that might have been crimes, but were not within the scope of his investigation. Michael Coen's violation of campaign finance laws by paying Stormy Daniels is an example of one such case referred to the Southern District of New York.
It is possible that Mueller did look at Trump's taxes and referred that to some other office. This would be unlikely to be revealed publicly unless and until a criminal indictment is filed. The standard practice of the Justice Department is not to disclose whether or not someone is under investigation for something or why unless they choose to charge that person with a crime.
I'd ask the question again in a few months; we might know something more then.
