Is there a way to know from far the order of pokemon placement in a gym?

The one sitting on top you can see from a distance is the last one in. Other than that, I don't know if/how you could tell.

When you click on a pokemon, you can see how long it has been there 0d2h3m for example. Anyway if the difference is 1 or 2 seconds, you cannot see which one is there before

Last one in sits at the top of the gym. Other than that you will have to look at the defending time of each month in the gym to know who was first.