I’d love to know how reconnaissance was handled. Aerial photography, obviously, but there’s literally thousands of square kilometers that have to be photographed - how did they determine likely areas to photograph, and how many people were involved in interpreting the images? And so many different uses for the information gleaned from these photos: landing sites, targets for strategic bombing, factories, transportation hubs, bunkers, etc… I also presume that photography aircraft were shot down fairly often?
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Reconaissance is a multi-layered matter. As you say, how do you know how to look?
In the modern era – WWI aerail photography was in its infancy, but commanders were basically interested in the front lines and a few miles back from there. There were a lot of two-man reconnaissance aircraft, and the fighters were there to protect their own and shoot down the bad guys’.
But by world war 2, there were a bunch of different sources to be used
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signals - radio could be ‘attacked’ several ways. Each could give clues to what should be looked at.
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signals/decrypt - if you can read the other fellow’s traffic, you know where and when things you want to look at are. Pretty much every country had mathematicians who could crack codes - the ‘enigma’ coders were the exception which needed a massive effort to crack, and then not even close to all the traffic read. One-time pads were great, but were typically not used for tactical traffic.
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signals/signal analysis - this is just a matter, with triangulation, of figuring out where a radio transmitter is. The germans had their u-boats surface every day and radio their position. The Allies could use that to figure out how many submarines were out there, and generally where they were. Wolfpacks could be avoided sometimes with this information. (“Radio silence” is to avoid giving things away to signals analysis.)
Another useful thing to do is count the number of radio transmitters you can identify. More transmitters, more units. More units, go look and see. (The pre-D-day “1st Army” deceptions were just a bunch of radio transmitters sending bogus (but still encoded) traffic to each other. That would make the Germans think there was something going on; the blowup tanks and other fake equipment would then be photographed by German photographic aircraft. Turned spies giving bogus ‘confirming’ reports completed the deception.
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Sound - if front line troops hear a lot of something (like tanks), might want to go have a look.
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Patrolling - Everybody patrols, to have a general look and see.
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Snatch Patrol - Go get one of the bad guys, and interrogate him/her.
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Spies - these are intended to get general intent, not specific locations. But if you know “they’re going to attack in the south”, that helps.
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Regular photographic missions - everybody takes pictures of everybody else every day. That goes into the mix.
All that informs you as to what to go have a very careful, thorough look at certain things. Lots of truck traffic going in and out of somewhere? Something’s there. Ships not where they were yesterday? Start looking elsewhere. Something out of the ordinary? Go look closer. Something you’ve never seen before? Go look closer.
All this winds up on the desk of the “intelligence officer” for the units of a certain size. It’s their job to figure out what they know, what they don’t know, and what they don’t know they don’t know.
… now, in WWI, photographic aircraft were awful. Slower, less maneuverable, bulky, hard to use cameras. They could be neutralized if enough fighters were put to the task.
WWII and on, though, reconnaissance aircraft had the benefit of great lenses, and great film. They could then fly much higher and faster (or, occasionally, kinda high but agile, like the Fw-189.) For instance, the P-38 had a photographic version, the F-5. The Germans had a recon version of the Ju-88, the Ju-188 which could fly at 31,000 feet. By the time interceptors could labor up to the reconnaissance aircraft’s altitude, they were long gone.
Nowadays, you can add satellite, airborne radar, sound analysis, radar of incoming artillery shells, infra-red photography, low-light-level photography, and all that gets added to the mix too.
Hope this (rather rambling) answer was of some use!
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Not merely “an” army: FUSAG! First US Army Group with multiple armies under command. The Germans appear to have been quite willing to believe that the “real” attack on Festung Europa would come from Patton’s FUSAG, and that the other landings were merely feints to draw off German strength from the “real” invasion. I’ve read in some sources that it wasn’t until the real Patton was appointed to command Third Army that the Germans gave up on the idea that there was another invasion about to arrive.
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You realize I could just edit my message and fix that. Then you’d look dumb… unlike… um… me. 
The Germans continued to believe the US had up to 80 divisions even after 3d army was activated - that’s how good the deception was. Patton’s role was to make the Germans think FUSAG was an offensive AG (no jokes, please), something they didn’t give up on even after Patton was ‘reassigned’.
One thing I didn’t know was that FUSAG was purposefully “made bigger” than 21st AG (commanded by Montgomery, including US troops) - on paper. That helped, I think, to reinforce the German estimation of jiahugic Allied forces in England.
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