-300: Normal wingtips -ER: Raked wingtips
-300: Smaller RR engines -ER: Bigger GE engines
-300: Engine sticker is a black square with RR on it
-ER: Grey GE sticker
-300: Normal 777 Landing Gear -ER: Semi-Levered Main Gear (includes an extra hydraulic actuator on the main gear)
(These stickers are Cathay specific and will differ for other airlines)
The other obvious way is knowing the tailnumbers.
For Cathay: B777-200: B-HNA to B-HND & B-HNL
B777-300: B-HNE to B-HNK & B-HNM to B-HNQ
B777-3ER: B-KPA to B-KPZ & B-KQA to B-KQB
Hope that helps.