If a Player on the field believes that the opposition is offside, they can call offside. If needed, they can use the perspective of those on the sideline to clarify if it was offside or not.
However players may seek the perspective of non-players to clarify the rules, and to assist players to make the appropriate call for “down” calls and line calls.
Line calls relate to the following calls: “out-of-bounds”, “goal” and “offside”.
So the player calling offside should not rely solely on the sideline player in making the call – they should have some idea that they were offside, and can then use the sideline as confirmation.