How can you tell if/when Otocinclus is eating?

[Amygdaland]

I have a new single oto in my 10g quarantine currently, who I picked up a few days ago before I realized they need to be kept in multiples.
I'm worried that he's not eating despite me dropping some bits of algae wafer in there. There's brown diatoms in my main 30g, but no sort of algae in the quarantine, hence the wafers.
I have no experience with owning otos, so I'm not quite sure what kind of behavior to look for to indicate feeding. I see him wiggling while inching backwards along the bottom of the tank. Is that him eating? He hasn't directly approached the wafer chunk, but there is debris from it throughout the tank.

[Autumnsky]

If you have a new tank, you need to feed him as you are doing, because the new tank wouldn't have food built up for him.

Have you seen him nibbling at any of the food when you have put it in or afterwards?

You can try a blanched slice of organic zucchini. Or a 1/4 chunk of it. You can steam or blanch the zucchini all at once and make little ice cup tray portions. One little guy will not eat too much. Remove uneaten foods after 8-12 hours.

You can always remove them sooner if you won't be home in time. Better sooner than after they start degrading. Often you can see a little green color in the tummy.

They are a touchy fish to get transitioned but after the transition, generally do well if they can eat and not be bullied. As you noted, they like a few friends. If you can a few more that would be great, And don't forget to give them a bit of a treat after they are in the tank to make sure they are eating.

One additional easy to feed packaged item is Seaweed Extreme marketed for marine fish, it has seaweed meal and Astaxanthin and vitamins. Many fish really love it. Especially bottom feeders.