Shemos ramban אהיה they dont need miraculous sign just the fact that H'is with us that he responds to our tefilah is the greatest sign of His existence & hashgachah. Mashgiach asked - so thats also a miraculous sign!
However ramban continues 'similarly' midah k'neged midah.
Meaning not miracle response but real tefilah can feel Hashem relating to you.
This is the great sign that yisro noticed as well. Ear of amalek showed a 'practical' G-d that relates to His people. Sh'mos.