The problem with checking an alternator after start is that the traditional instrument on aircraft for the generating system is an ammeter.With an alternator the ammeter alone is of little use. A voltmeter is much better and that is why cars and trucks have had voltmeters as standard for a long while.
On 12 volts the normal alternator output should be 13.9 to 14.5 volts. The higher the voltage output the greater load that it is handling. So a high voltage continuously may indicate a battery problem.However a low alternator amp output doesn't neccessarily indicate that the battery is happy.
Generators on the other hand tell all via an ammeter.
Its a mystery as to why aircraft which use similar alternators to automotive stick to ammeters.
Wunwing