In Battlefield 4, how do I know when I've been issued an order?

Solution 1:

Say you are playing Conquest. When someone issues an order you will see something like this over the flag identifier:

It will be an attack order on an enemy flag or objective and a defend on your own flag or objective. So you will see the symbol when you look towards the flag/objective in first person with the identifier (letter) with those marks around it.

Additionally in the mini-map view you can see attack/defend orders that look like this one:

Objectives are not available in every game mode so you will not see attack or defend objectives in those modes. One for instance is Team Death Match.

Solution 2:

In addition to what Dupree3 said, there will be text in the middle of your screen that says Order Received (though this will disappear).

You can also look at your mini map to see the dotted line. This shows there is an order pending. (waiting for you to accept / decline)

I will provide screenshots of this later today.