Hi! When measuring an ASK signal on a 'scope, how do I know if the signal is AC riding on a base DC, or 0-based AC? If I switch the coupling on the scope probe and trigger between AC and DC, the signal looks like same, except in DC, it's offset by about 4.5V and never goes below 0V.

 

It sounds like it is DC coupled.
If it was AC, the signal would swing around 0V, not always be above it.