Never throw anything away until you get home, is my advice uttered nearly every day.
In this circumstance then the first thing I will ask to see is the e-mail of booked online - if that confirms the trip, then I'll accept it.
If the trip involves multiple changes / reservations, I'll look at the other reservations and work out (using Avantex if need be) would they have been told to get this train.
If I have to go ringing Telesales to see if I can get an answer, then we are hitting dangerous territory. If I am wasting that amount of time, and they turn out NOT to be booked on the train, then any hope of an excess has gone out the window, and the possibility of a full open is looming, as they have tried to commit fraud (and worse, buggered me about). This has happened on several occasions when people have been in suits and "I can assure you young man, I am booked on this train, are you calling me a liar" has been their initial approach.
Sometimes (only sometimes) I have a reservations list, and if it is an "unusual" reservation I can look it up. The list usually doesn't have a name or ticket number, but if it is, say four people from Reading to Neath, I can look up the list, see if that is on the list, and if it is, check the booked seats to see if anyone turned up. A single from Pad to Cdf will not work with this, though!
The other thing I will do (although NOT obliged to, I hasten to add) is offer to issue a UPFN (full price), that will be cancelled if they print off the e-mail when they get home and send it back with the UPFN OR pay the walk-up price now and that's the end of it.
The genuine ones take the UPFN, the chancers pay the super-off-peak.
And DodgyDave, don't blame the guard for your own inability to get your act together. He sounds to have been a lot more competent than you.