on a character which hasn't gone to cyro that campaign, repair a wall or heal someone at a keep.
when you have gained AP wait for your char to register on the leaderboards - this will show you roughly how many players maximum there are and give you some idea of how many you need to be in top 10/2%
That's the worst workaround to guess it. Very impractical, if you are active in multiple campaigns. In other words you can't really see it without that trick.
You can check your current rank in a campaign in 2 ways (although it will not give you the actual top percent; ie: top 2%-3%).
First method. Click on the Caslte icon for the Alliance War (or press "L"), make sure you are on the Overview tab (It should sow the score). Click on "Emperorship". There it will show the top 10 as well as your current rank.
Second Method. Click on your Journal (quest log) ("J"). From there click on the far right tab which will be "leadership". That will allow you to see the top 100 players. It has an option for Overall (top 100 players of all 3 factions combined) as well as options for the top 100 leaders of each faction.
thank you, but this is not related to the campaign rewards. They are calculated for the top 2-10% of each faction for each campaign. The method of Izanagi.Xiiib16_ESO seems to be the onliest way.