Getting your eye in
From Austin Storm
Introduced to me as a term from mudlarking, it means being attuned to tell-tale signs of 'otherness'
Maybe it's just a British term, because here it is in an article on spotting IEDs:
“We were very experienced and had been acclimatised to the heat. We’d got our eye in. We knew what bumps and irregularities to look for. After immediate deployment I spent weeks training in the test lanes at Bastion, looking for the smallest sign of a man-made disturbance, a single raised stone or unnatural discolouration, anything to indicate a bomb.”