The constant lying and backtracking (about giving to charity, about the finances of his campaign, about people 'celebrating 9/11' in New Jersey), his massive outlandish fantasist policy promises, his penchant for surrounding himself with extraordinarily corrupt people (Paul Manafort, Roger Stone, Michael Flynn, Scott Pruitt, and yes Michael Cohen - the man who was his personal lawyer and vice chair of Trump Org for 12 years who everyone is now trying to distance from Trump). Trump came into office promising to 'drain the swamp' and all he's done is build his own swamp. Then there's the evidence that Trump has used his position as President to enrich himself and his family (foreign dignitaries and lobbyists suddenly spending heaps of money at his properties since he became president, foreign governments assisting expansion of his businesses abroad, using public money on his own hotels and businesses etc etc etc). Put this together with his long history of unethical and shady business behaviour (I'm thinking here of things like the Trump University scam), and it really is not difficult to conclude that this person is an obvious conman.