The best way not to get poshed is to get the first round in. Then later in the night, when your compadres are drunk you can seagull a few drinks from them.
How to realise that you are getting poshed: when it dawns on you that the content of your wallet will either stretch to a curry and a taxi home, or one more drink - and you get the drink.
A popular place to get poshed is a distant cousin's wedding where the free bar has been withdrawn. Thus you end up drinking your taxi fare back to the B & B and having to scrounge a lift from some pissed-up arse in a Range Rover.
'Nah, I'm totally poshed mate. Lend me a tenner?'
'Good night out?'
'Yeah, we went up town now I'm completely poshed. Lend me a tenner?'
'How was the wedding?'
'Crap, the freebies were denied so we got well poshed!'