So let's unpack a few things right up front... 1) He left California and was in DC at the hotel where the WHCD was being held.

2) Imagine the intent he had to have in order to travel that distance all the while thinking about what he was going to do. Eric Schwalm@Schwalm5132

3) He either moved several weapons or procured them in/around DC and figured out a way to get them in proximity to the WHCD. 

4) He took the time to load the weapons on to his body (to reportedly include several knives) and nobody noticed. 

5) Now here is the kicker...in his hidden location he was somehow aware that the President was in the room and was seated. How/when was he aware of that part?...