How most people figure out if someone's story is credible  

Trump is mounting multiple defenses about the Maro Lago invesgation. He's using all the catch word phrases 

"This is politically Motivated"

"Why were Obama and Bush not investigate...They did the same thing"

Let's analyze the last statement

The National Archives and Records Administration (NARA) explain in August that Obama di not take the presidential documents Trump claimed. Rather, NARA itself moved the documents from the Obama administration to a NARA-managed facility in the Chicago area, near where Obama's presidential library is being built. Same exact scenario holds true for Bush

Given Trump was caught in another bombastic lie, does that not diminsh his credibility once again?