My brother seems "posh". Yet we are from a very underprivileged family. We often joke he was born into the wrong family, so I am not specifically talking wealth.
He was the first to go through university in our family. He has a very high paid job but his wardrobe is very modest. He will buy a few expensive, well made clothes which you will see on his back for years to come. He hates to part with money at all and has savings coming out of his ears. Although being mid 30s having no issue affording buying his own property perhaps 4 or 5 times over, he has never moved out of my fathers from his very modest bedroom - 90s threadbare carpet, single bed. His tech isn't flashy either. He has almost no desire to keep up with anybody and is super conscious of the environment, he doesn't preach but likes to do what he can and takes good care of his body - resulting in very clear, young looking, blemish free skin and good teeth. He has always listened to classical music since a child and has always been super well spoken - he was always a well behaved lad, he didn't hang round us street kids like me and my other sibling, so picked up very little local dialect and slang. He talks from the base of his stomach rather than from throat, so he has a plumy, rich, strong voice. - I see this on alot of "posh" people and I am not sure why this is. He also crosses his legs and sits down like a posh man. As someone who is from the underclass, very often someone who has a whiter than white, goody two shoes image can also fall under the "posh" bracket.
I also find the more posh somebody is the more they adhere to their stereotypical gender roles.