Dwell meters readily available.
Unless you really enjoy math and like compensating for fluctuating system voltage - buy a dwell meter.
Click to view attachmentAvailable in stock in Autozone in my neck of the woods but much more expensive
Click to view attachmentTrying to measure with a digital multimeter is going to be hit or miss depending on what type of averaging it does to the DC voltage input.
But here you go:
Let’s say your battery voltage is 14v with the vehicle running. (Humor me - makes the math easier)
Dwell for a 4 cylinder is approximately 360/4*(measured voltage/system voltage)
So let’s say the meter reads 7 volts engine running. Black lead to chassis ground, red lead to coil (-) negative.
360/4*(7/14) = 45 degrees of dwell
This is much easier done with an old analog multimeter with a good old fashioned low pass RC filter on the DC input. You’re likely to have issues with the digital meter not reading properly and not averaging correctly as the input voltage is jumping between 14v and 0v and also being subjected to high voltage transient spikes from the coil fly back.