I must now have an 'I know what I'm doing' look as it seems to happen more often now. I think at the station I work at, people recognise me so ask anyway, but how it happens elsewhere I don't know. Was in SWT land yesterday wearing a blue t-shirt that vaguely resembles SWT/EMT colours (but is a bit brighter) I got asked questions a couple of times. One guy actually apologised saying "I know you're off duty but..." - as it was a Clapham Junction I had to refer him to station staff, that place confuses me too much to give him an accurate answer.
Ditto for CLJ
Though I think I know what you mean about the "'I know what I'm doing' look" - if you work on the railway but are riding around for leisure would you... carry yourself differently to how you used to? I would have guessed so. Unless you try to hide it!
I kind of like platform 13 because the throughput of trains is such that nobody ever notices how long you've been stood there but it affords a great view of proceedings - perhaps I do look like I'm 'working' because I'm checking the train and the signal and usually standing next to the guard. In fact, last week when the R indicator was broken, I had to resist the temptation to shout to some of the guards (generally the non-local ones) who didn't know to use the buzzer!
It's never happened to me. Perhaps because I don't dress smartly.
In fact a lot of the times when I've tried to help someone who looks a bit lost or confused, they seem to take offense that a member of the public thinks they need help.

Don't worry I don't dress up to go bashing. Though now you've given me the idea...
I often see people, frequently foreign tourists, whom I know I could help... but don't. For one I might
not know the answer (that'd be awkward!) or I might not be able to understand them... but mainly you can never predict how anyone might react!
I've had more than one passenger wanting to be mistaken for staff, wearing quasi-uniform, high-vis, "look like a freight driver at first glance" type clothes, invariably they didn't have a ticket.
I once took a FGW lanyard off a chap who when asked for his ticket pointed at his lanyard - him and his mate (both teens) had drivers bags as well, it was the shorts and flip-flops that made me suspicious!
Innovative faredodging technique? Flamingo's busted it! All we need now is for 455Driver to turn up and accuse me of impersonating staff!

Though to the guy who needs a more expensive suit... LOL, though I think the FGW uniform is one of the smartest. Not because it looks inherently better, but because you might actually wear those colours. How many people really wear purple shirts or maroon jackets?
Methinks ATW have a fairly greenish uniform? But do they still have grey trousers/waistcoat? And LM's shirts are a very odd colour!