This is a hard one to answer.

What is really needed is a "do not disturb"/"OK to wake" sticker. I believe these still exist on some carriers/routes, though they used to be more common.

Quite often I will doze a bit onboard but I DO want the drink, snack, and/or meal. Luckily I usually wake up from my dozing in time to get it. The suggestion to use your tray table to signal your intention to be served is a good one.

It isn't always easy to "read' other people's intentions. Of course, if someone is using a pillow, blanket, earplugs AND eyemask, that might be a tipoff....