Step one: does it leak when the shower is off?
Dry everything, then leave the shower unused for a full day and check for fresh water. Water appearing with the shower off points to the supply side - a valve or a pipe joint - and is more urgent.
Water only appearing during or shortly after use points to the enclosure: seals, grout, tray, waste or tanking.
Step two: separate the tray from the walls
Block the waste, run water into the tray and leave it standing without running the shower. If water appears below, the leak is the waste or tray connection.
If that test is dry, run the shower onto the walls only, avoiding the tray, for several minutes. Water appearing then points at the wall seals or the tanking behind the tile.
Step three: read the evidence downstairs
A ceiling stain directly under the tray edge suggests the tray or waste seal. A stain offset towards the door usually points to water tracking along a joist, which can be a long way from its source.
Photograph stains with a date, because the rate of change is useful evidence for both the repair and any insurance claim.
Realistic repair costs
Re-siliconing a shower is an hour or two of labour. Replacing a waste and re-bedding a tray is half a day. Failed tanking behind tile is the expensive one, because it means removing tile and rebuilding the wall - which is why it is usually the point at which a shower refit makes more financial sense than a repair.