My DS had a hearing test around this age (he too had suspected ASD at the time, now confirmed).
The test they tried involved a toy piggy bank, and he was expected to put a coin in the pig every time he heard a sound from the audiologists hand held machine.
He couldn't follow the instructions, so they gave up, and asked him to come back in 6 months. They also did a glue ear test which involved putting a wire in his ear
When we went back same thing happened, so they invited him back AGAIN but this time they did the test they use for babies, which involved a screen at either side of the room with a visual that appears at the same sound as the sound, and they wait for the child to turn their head left or right, depending on the direction of the sound.
He passed this test, but they kept inviting him back, in the hope that he would be able to complete the original test with the piggy bank, because they said it was a better test (could test for a bigger range of sounds or something?).
He never did pass the piggy bank test because he couldn't concentrate for long enough, and they eventually discharged him when he was 4.5. He then failed the school hearing test (for the same reason!) so he got referred back...
Needless to say there doesn't seem to be anything wrong with his hearing, but he has clear attention and understanding difficulties which were the biggest barrier, and I found the whole test thing v frustrating (as did the audiologists I'm sure!).