I remain of the opinion that it is fairly simple: look for the info plate and check it matches the bike at first glance. Most of the ones you need to worry about aren't even going to fake an info plate.
Some of them should be barred for having bulging or cracked batteries, whether or not they're attached to a legal bike.
The staff would see that as an argument with someone who is possibly a nutter
Yes - they can spot the dodgy ones in many cases
but if they try to stop someone getting on a train with it then if they are right then the "customer" is likely to argue
and they just don't want all the hassle that can cause
so they keep it very simple and easy to prove quickly and without having to check plates - which are probably available on the WWW anyway
mine is totally legal - but the plate looks like a sticker - allbeit a good one