Actual Volumetric Efficency

Specifically, when doing street tuning. What can be used to measure the actual airflow for calculating VE? Would a MAF value be a good instrument for this?

The actual VE is the measured airflow divided by the displacement of the engine. So yes you could do that with the right formula. A lot of stock ECUs do that automatically through their various engine load calculations. However when you are populating a VE table for purposes of road tuning and achieving target AFR's, it's not as much of an exact science. There's a certain amount of error and fudge factor built in.

On GM engines for example (HP Tuners type of tuning), you basically have a VE table and a MAF scaling table, and they are adjusted according to the o2 sensor feedback.

VE is typically measured using an accurately calibrated airflow meter on an engine dyno. The reality is for us outside of the OE tuning world we're probably not going to be able to accurately measure the VE of the engine. That's ok though because it isn't as critical as it might seem.

When it comes to tuning in the aftermarket world, we're really manipulating VE values until our measured AFR matches our target. Particularly with aftermarket standalone ECUs where a lot of ECUs are now offering VE fuel models, the absolute numbers may not be completely accurate as there are so many other parameters (injector characterisation for example) that can end up impacting the final fuel delivery. Take the numbers with a grain of salt, understand the tuning principles involved, ensure you have the basics covered and configured correctly and then adjust your VE table until you achieve your target AFR.