Ideally, you do need to get nearby AIS receiver/transponder to check the sentences sending from yours. But checking the TX LEDs on A051T gives you an idea that the internal transmission module works well. After powering up A051T, within 10 seconds you should see the TX LED flash twice. This indicates that A051T transmit an AIS sentence in two channels respectively.
In practice, if you have an AIS receiver, e.g your A023, which is less than one meter away from the A051T, you probably will see the AIS sentences on your A023 even no VHF antenna installed on A051T.
Good luck.