As I was expecting the US ISR can detect mobile missile movement and electronic chatter that comes with any military moves including moving VIP's to secure locations. Point trying to make is the US will know when china has decided to attack Taiwan with Ballistic and cruise missiles alone. If Houthis are also moving missiles into position to synchronize a launch with Iran US ISR is likely also detecting it and when it comes to Houthis the US should be blowing up their missile launchers.
The margins for detection of actual intent on the Taiwan front is likely to be lower though.
The PLA routinely activates large numbers of aircraft & ground/amphib assets to operate in close proximity to Taiwan as part of their "drills/exercises" that any such build-up might slip under the threshold of what might be considered 'normal'.
Of course there's likely to be ELINT intercepts to back up any intel assessment but the question will be whether it'll be enough to convince the higher-ups that this build-up isn't like all the others.
The Taiwanese have learnt to closely monitor any & all such activity, however - something tells me it won't be hard to convince them. Not to mention, ROC has significant intelligence penetration of PRC at a HUMINT level.