Surely jockeys have the use of some sort of scales in the rooms to figure out their weight before going out to be officially weighed.
Otherwise they'd all be jigging around with gear and weights etc at the official weighing and it would take forever.
If that's the case they'd all be noticing the difference wouldn't they?
From my experience there is a set of scales near the jockeys room which they weigh out on in front of the clerk of scales.
Jockeys know pretty much what they and their gear weighs so it is a matter of possibly adding or subtracting a small peice of lead from the bag to weigh out correctly.
Weighing back in is done in front of the stewards and quite often on a different set of scales.
Mind you only the first 5 weigh in.
The 7th horse could have had an extra 20kg on its back for all anyone would know