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We'd probably have some idea the race is on. Telltale signs like earthquakes, changes in caldera elevation, volcanic gas emissions and temperature changes would offer advance warning.

Here's the weekly report for the world's largest active volcano: https://volcanoes.usgs.gov/volcanoes/mauna_loa/elevated.html

>Would we at least know this is happening?

Lots of earthquake activity in the area, as I understand it.

Lots of smaller earthquakes is what to look for.

What about local changes in elevation? If there's a ton of new material in the crust would we observe deformation in the caldera?

Yes. In fact, scientists track the Yellowstone caldera's altitude and watch it "breathe", so to speak.

https://www.yellowstonepark.com/things-to-do/caldera-rises