Is it possible to determine if a Pokémon is a powerful encounter?

It is not really possible to tell if it is too powerful without engaging the battle. Once you started the battle, you can tell by their level.

The ability to catch higher level pokemon is tied to the number of gym badges you acquired so far. It goes as follows:

• No Badge: up to level 20
• First Badge: up to level 25
• Second Badge: up to level 30

this goes on in intervals of 5 levels all the way up to 8 Badges, at which point the level cap is removed.

The pokemon can spawn with different levels as well, take your Onix encounter as an example. Consider you having 1 badge meaning you could catch Onix up to level 25, you might encounter one that is level 30 or higher, meaning its too powerful. However there are also lower level Onix spawns as well, say you encounter a level 10 Onix instead meaning it would not be too powerful to catch.

In-game I found a note stating the following:

The large Pokémon you see wandering around in the wild area are strong! If you try battling one and it seems like you're in over your head, try to run! Run like the wind!

This note dropped right in front of a giant Onix (well, all Onixii are huge) but it was indeed a powerful one (I was level 12, he 26). So I guess it is visible on beforehand, if the Pokemon is larger than usual, but I have not been able to compare it myself.