The signals on an antenna are RF signals which are AC. You'd need a service monitor or an oscilloscope with micro-volt scale.
You can get a ballpark figure if you have a decent communication receiver with an S meter. The S meter generally shows how many micro-volts are at the antenna input. I say generally because tests have shown that no two receiver brands are exactly the same. In some cases there can be big differences.
...but the hard part is....how do you calibrate the antenna?
The "S" meter on a good radio is the obvious choice. There are some massive signals out there, but none you can measure with a digital voltmeter. A lot of shortwave broadcasts are S9+20dB but that's still only 500microvolts in 50ohms.