Get a strong magnet (neodymium) and dangle it against the wall. It will be attracted to nails or screws. Mark them with a spot of coloured chalk (you can clean it off afterwards).
Probably they will be in vertical lines, about 600mm apart. Once you have found a few, you will easily find the others. You may be able to feel or see the slight dimples.
Depending on method of construction, there will probably be wooden or metal uprights fixed to the dry side of the wall, and an air gap. The tanking will be behind the air gap. The metal method may have special waterproof fixings.
You need to work out what the construction method is. It may be mentioned in the building regs application or the builders quotation.
You must not penetrate the waterproof layer, but you can fix to the wooden studs or metal lathing.
If the build has no air gap or studs, you probably can't drill into it at all without reinstating the waterproof layer.
A really good conversion will have an entire new wall on the dry side, with a drained and ventilated gap.
An older or cheaper method may have a waterproof coating painted on, blinded with gritty sand, and plastered onto the sand. You can't drill it, water will come through.