Ask how the work is sequenced and who does what
A fitter who can describe the order of work, name which parts are subcontracted, and say who issues electrical certification, has done this properly before. Vagueness on sequencing is the strongest early warning sign there is.
Ask specifically who is waterproofing the shower area and what system they use.
Insist on an itemised quote
Separate lines for strip-out and waste removal, plumbing first fix, electrical work, tanking, tiling labour and materials, sanitaryware, and making good. One number on one line cannot be compared to anything and cannot be held to.
Also confirm what is excluded - unexpected joist repair and plastering are the usual exclusions and should be stated, not discovered.
Check the practical things people forget
Confirm working hours, parking, skip or waste arrangements, whether the water will be off and for how long, and whether the property will be left usable each evening. Confirm dust protection and floor protection to the work area.
Agree how variations are priced before they happen, in writing.
Payment structure and red flags
Stage payments tied to progress are normal. Full payment up front is not. Cash-only with no invoice is not. Pressure to decide immediately on a discount that expires is not.
Other red flags: no written specification, reluctance to show tanking before tiling, and no clear answer about certification.