Visual, I believe.
If there’s a hook echo on the radar, they’ll try to verify it that way.
If there’s significant wind damage somewhere, with a possible tornado, the NWS will send someone out to look at the damage pattern and try to determine if it’s the circular type that indicates a tornado, or the straight type that indicates a gust front or downdraft.
But they just count them by report to the best of my knowledge. There are likely more tornadoes that have happened than are counted, simply because the criteria for verifying them is rather stringent.
If the count was correct, this will be the record for the most tornadoes in a 24 hour period. (160)
There are, BTW, career weather experts . . . scientists types . . . on my side of the argument.
I don’t have any links handy.