You may take into account the units selected for deployment by players with a turn order before yourself. So, for example, if you are the second player in turn order, you are only permitted to get information about the first player's selected units, not the third or fourth.
Deployment stacks themselves are created simultaneously and privately. You can use your hand to obstruct while making your deployment stack. So, your opponents may know what units you have selected, but not the order in which you are deploying them or which units have been grouped. Only once all players have locked in their deployment stack is the stack revealed. The top unit is open information, and some information about grouping will be evident from looking at the stack, but you are not permitted to look through each other's stacks and see the order of your opponents' units.