The leaders generally know where their own troops are, and they’ll order them to attack places where they think the enemy troops are. After that, troops generally find ways to identify each other by uniform and equipment. When they have time, they’ll use passwords and such, or ask for names and unit numbers that the enemy wouldn’t know.
There was a case during WWII where the Germans had a much better submachine gun than the Allies, and a group of Allied soldiers captured a supply depot and took a bunch of them. Later, an Allied mortar company heard the distinctive sound of the German guns firing close by and dropped mortar bombs on them until it stopped. The system doesn’t always work.
Today, countries do their best to make their uniforms different enough to tell, and they try to stay in radio contact with every other unit nearby, because neither side wants their people shooting each other. It will still happen, because war is chaotic, but they try.
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