What tanking actually is
Tanking is a continuous waterproof membrane applied to the floor and walls of the wet area before tiling, sealed at every corner, junction and penetration with matching tape and collars. Tile and grout sit on top of it and are not themselves the waterproof layer.
Because a wet room has no tray to contain water, the membrane and its detailing are the entire defence.
Liquid membranes versus board systems
Liquid-applied membranes are painted or rolled on in coats and are well suited to irregular shapes and awkward detailing. Sheet and board systems are bonded in place and give a very consistent thickness, and are often used with a former to build the falls.
Both work when installed to the manufacturer's system. Mixing components from different systems is where warranties and performance are usually lost.
Falls, formers and drainage
Water has to reach the drain. That means a graded fall built into the floor, usually with a proprietary former in a timber floor or a screeded fall in a solid floor. Getting the fall wrong leaves standing water and, over time, staining and smell.
Drain choice matters too - the trap has to be accessible for cleaning without lifting tile.
How to check the work before tiling
Ask to see the tanked room before tile goes on, and photograph it. You are looking for continuous coverage, taped internal corners, collars at pipe penetrations, and coverage taken high enough up the wall and far enough beyond the wet zone.
A fitter confident in their waterproofing will expect this. Reluctance to show it is a meaningful signal.