Pokemon-go – Is it possible to tell the duration of someone else’s lure

The game, as of now, does not have a builtin direction indicator.

You can use the footsteps to determine which way is approximately the right way to go (if the count gets larger, you're going in the wrong direction), but the game is intentionally vague on this front.

Borrowing from @Codestrife's answer here

The rustling leaves usually indicate that there are pokemon near that area. Going and walking around near that area gives you a higher chance to encounter Pokémon.

Keep an eye out for rustling leaves, hinting at where critters may be lurking and simply hang out in a suspected region for the increased chance of an encounter. - Source

Therefore, the reason you don't always see Pokémon in rustling grass is because there is only a higher chance to encounter them there and not a guaranteed chance of encountering them. You just have bad luck when you don't encounter them.