What is "loop check/test" when applied in electrical works?

Answer 1

Former responder is correct as far as he goes, but a loop check can be applied to any circuit (power or data) to verify that the wiring is continuous. If you short the conductors at one end, and apply a potential at the other, the potential should appear on all the connected conductors (usually, you're working with a pair, so there is only one).

Answer 2

A echo check or loop check is to determine the integrity of transmission of data, whereby the received data are returned to the source for comparison with the originally transmitted data.