How is current measured?

The basics: current is measured with an ammeter in units called amperes (amps for short). You will want to look at continuous and peak amp performance specs for the particular model you are interested in. It is important to discuss this issue with the speaker manufacturer as well. They should be able to provide you with a guideline as to what will do the job.

Then you should be able to get started on selecting an amp that will not only have enough current to drive the speaker, but also sound good in the process.

You can examine maximum continuous current by looking at specs of output devices and applied collector(or emmitter for emmitter follower) voltages.
Measuring the current with Ammetter won't bring you a sufficient information unless you're using an oscilloscope to analyze transients.

I will give you some examples how current is related to the power rating of the amp and to the speaker load. You can determin how much current (I) your amp can supply from the spec of max. Power (W) into specified load (R). W=I^2 x R. So if the spec says 200W into 8 Ohm and 400W into 4 Ohm means that whan load drops and more current is needed your amp can deliver 400 divided by 4 = 100, sq. rt. of 100 = 10 AMP.
However, if the spec says 200W into 8 Ohm and 300W into 4 Ohm - using the formula above we only get 8.66 AMP from the amp when the load drops and more current is needed.
Be aware that if the amp spec only says 200W into 8 Ohm load - and nothing else - the calculated current of 5 AMPs may be close to the amp's limit and when load drops let's say to 6 Ohms - the amp will only deliver 5x5x6=150 WATTs of power insted of rated 200 WATTs.
I hope this helps.

Hi. Using your calculation and applying it to my Plinius 8200K1 rated as 175-watts @ 8-ohms and 250-watts @ 4-ohm gives a current of 7.9 Amp.

I was thinking of changing up to the 8200MK2 rated at 175-watts @ 8-ohm and 300-watts @ 4-ohm giving it 8.6 Amp. So is 8.6 Amp significant in actually listening to 7.9 Amp?

btw, I am using the Magneplanar 1.6QR. Thanks.