It's as simple as checking the fuel cap - if the cap recommends RON95, use RON95, if it says RON98, use 97/98/99.
Some people think higher RON fuel is more calorific, its not. The more powerful cars need more fuel per combustion cycle to derive that power. You cannot get enough RON95 in per combustion cycle to generate 300+ hp without it pinking when compressed.
Higher RON fuels are less volatile, so you can inject more without fear of pinking. The likes of my S3, the Golf R and very high outputs of GTI editions are recommended RON98 because otherwise they would be limited on output. When these cars know they're running on RON95, the engine management limits output to about 280ps, because that's as high as you can reliably get with RON95 fuel and no pinking. If you have a 245ps GTI, there's no need to feed it 97/98/99, apart from the possibility of having more detergent additives in it.
I usually use RON97 from Costco for my S3, but I have been away with work, drove Newcastle to Heathrow for my flight. By the time I got my car back from Heathrow external parking, it was 10pm,Watford Costco petrol station would be closed, so went with Tesco extra 3 miles from Heathrow to fill up for the journey home. It's Momentum 99, made absolutely no difference to the car's output or mpg having RON99 vs the usual RON97, the car can compress and combust both without pinking.