There is a lot of speculation as to the source of information for this wiretapping claim (conservative radio host, old news articles only tangentially related), but the reality is all Trump needs to do is make a call to verify because he has all the clearance and authority to get an immediate answer. And if it did really happen, he should launch an investigation. Tweeting about it without proof is ludicrous, and ultimately inexcusable without proof, but that's par for the course.
In fact, FBI director James Comey, was so appalled at this statement (the FBI is the one that would do the wiretapping) that he requested that the DOJ immediately issue a retraction on this statement, noting that Trump didn't even ask him if it happened. Crickets so far.
Shouldn't we hold our President to higher standards than this?