@elaeocarpus
I think the main reason for grommets is preventing ear infections and fluid build up, rather than improving hearing as the primary reason for grommets ( i think?).
That wasn't the case when ds had grommets. Ear infections alert you to them needing to hear, so helps get a referral, but they don't do grommets to stop ear infections. In fact they were very clear that grommets don't always stop ear infections; it's a nice side effect. Ds was getting ear infections about every 10-14 days so it was good not to have them, but they wouldn't have done it if his hearing was fine.
His friend who had ear infections but they drained quickly and didn't effect his hearing didn't get grommets.
Op, they can do a sort of hearing test by getting them to put a toy into a bus or similar when they hear a noise, but it's pretty random until they're old enough to understand. For ds that was about 3yo, although at 3yo they found that he thought putting a person into the bus was totally boring so he ignored them and just played with the bus. They then tried the button, which he loved, so concentrated well.
What they did do for ds when he was tiny was a puff test. The graph should have a nice peak on the line. Ds' were flat.
Ds had grommets at 20 months, 3 years and 6years.
I never had the immediate "amazing" any of the times. The main thing I noticed was the change in not having ear infections.
But ds noticed things. The two that stood out was him telling me that "the leaves didn't crunch last year" ie he couldn't hear them when he was 3yo, and when he was 6yo him rushing out of school to tell me the exciting news that "sh" and "ch" made different noises - before the operation he couldn't hear the difference.
However the difference in the hearing test when he was old enough to do one properly was he went from they said less than 10% hearing well into normal range.
But most of the things were not that noticeable at the time. I can look back and think "oh yes" but at the time I didn't see them as they were subtly better.
The watching your mouth is exactly the same as ds. He had this cute little habit of when he was talking to me of patting me on the cheek. ENT pointed out this was him turning my face so he could lip read. He also wanted to be carried - so his ears were near my mouth.
He had a less cute habit of telling people what their PIN number was - you'd be surprised the number of people who mouth it as they put it in.
He stopped turning my face while he had grommets in - in fact it was one of the ways I realised his grommets had come out the second time. He'd adapted so well that it wasn't very noticeable most of the time.