"Lying off" from a port awaiting fair conditions, enough water, permission for entry or some other requirement, is still quite common. Even now when many vessels have sufficient power to overcome most adverse tidal flows a river mouth will often be dotted with craft waiting for something before they enter. Unlike planes which must keep moving or fall, ships will seek sufficient water to be safe but shallow enough to allow them to anchor.

Shields bar is mentioned in Here's the Tender Coming as a place where vessels lie off. I haven't checked on a chart, and don't get into those waters myself, but your sandbar to the south of the Tyne sounds like a good candidate (S. Shields being right on the river mouth, south side)

Knowing where the bar is would be down to a variety of things:
1)Wave motion changes visibly over shallower water, mariners lives depend on noticing such things
2)If conditions are good one may be able to see bottom (not a safe one to rely upon, but a good confirmation)
3)Local knowledge of the skipper and crew, many ships running a habitual if unscheduled route would build an impressive knowledge-base
4)Charts, used in conjunction with compass bearings to triangulate the vessel's position
5)If "feeling" for a particular channel a leadsman in the bows would help
6)Most importantly, pilots with good local knowledge have been a feature of the Tyne for around 500 years UK Pilotage history.