What i look at is the sealer between the joins say of the shower tray and where the walls join each other.If you notice some of the sealer has perished or gone then its a sign that you may have water ingress.
I noticed new sealer on a caravan we recently saw and this made me suspicious and it made me wonder if the seller had just sealed it to make it look good and he had in fact just sealed water in behind it!
How can we test behind all of that plastic and solid backed over head lockers for damp? the professionals obviously have a way of checking it out or use special tools but I can't find anything about it on the internet
Our service damp reports have a hatched out area with "Shower" written on it and no readings - so I don't think the area gets tested. Best thing is to check the floor, ceiling and walls as close to the shower as you can get, then check the sealant outside for gaps, dribbles etc. Stick the probes into the underside beneath the 'Van round the shower drain and floor to wall join too maybe.