I would suggest using a clamp ammeter that supports DC measurements too. They have measurements up to quite large values, for example my UT203R supports measurements up to 600 amperes and can measure currents as small as few hundred milliamperes with an acceptable accuracy.
Not every clamp ammeter supports DC, some support only AC. So choose carefully.
Find a good model that supports DC, start your vehicle, let it run for half an hour to ensure the battery is fully charged (or else you measure charging current too which you probably don't want to measure), then zero the ammeter with a button (needs to be done due to stray magnetic fields) and put the clamp ammeter around the alternator positive wire. It has to be around the positive wire only, if there's a separate negative wire you can't put the clamp over both wires at the same time or else the currents cancel out.
It should give you a pretty good indication of the amperage.
Set the electrical devices as you wish (e.g. lights off, stereo on if you want), but you probably can't do the measured when driving on the highway so simulate that by asking someone to rev your engine to cruising RPMs while you do the measurement.
