I put my bag on the seat during rush hour*. That's because I've chosen a nice quiet commute, where there are plenty of spare seats. I'm no train n00b, I've been commuting by train since I was 11. Its not really any indication.
*I do move it of someone is looking like they might want to sit there
As do I - I always sit in an aisle seat, because I simply cannot stand being trapped between the window and someone else, and frankly because I don't think most trains have enough space for someone to comfortably be able to do anything other than sit with arms in lap.
If I have a bag, I will put it on the window seat purely for convenience purposes.
In either situation, I will happily move my bag and/or get up to allow someone to occupy the window seat,
if they ask. I simply cannot stand people who steadfastly refuse to ask and just stand there looking like a lemon.
Funnily enough I seem to quite frequently end up in a comfortable seat with nobody sitting next to me - probably because of the unique British 'I will not talk to anybody on public transport' thing...
Of course, obvious exceptions to the rule are all those people who should have been offered the priority seats - I will always offer to move for them and if they are travelling with someone I will happily stand to allow them to sit together.
Back on topic, you can usually identify someone new to the UK railways because there'll be a half-mile long queue behind them at the TVM. They'll also probably not be travelling on the correct train for their advance ticket, or will panic run to the platform the second it is announced!
At Gatwick you can easily spot people who aren't used to UK railways because they'll be accompanied to the TVM by someone in a purple London Transport Hi-Vis jacket to be sold the most expensive possible ticket to London. Then on the way back they'll be accompanied by gateline staff to a bank of Southern RPIs ready and waiting to give them a penalty fare for using Oyster because Gatwick isn't part of the London Transport system....