Apologies, this is long and a bit ranty.
I've been a Presiding Officer in a polling station all day. Let me tell you about this.
Yes, technically you could find out how someone has voted. You are assigned a voter number, we then write that on a list where your number is put beside the number of the voting paper you are given. To find out how you voted, someone would have to have access to your ballot paper, the list of matched numbers, and the voter list. The latter two are kept in separate sealed envelope. So, technically, votes could be checked back for fraud purposes I guess, but it would be a long and complex process. No-one would be able to 'sneak a look' ad hoc though.
Secondly, there is just no way that pencil votes can be rubbed out and redone. This is lunacy. Firstly, as PO, I've got my eyes on the sealed ballot box all day. I seal the wee letterbox bit at 10pm. It then gets put, fully sealed, into a van which is following a pre-prescribed collection route round a group of polling stations. Every Presiding Officer on this route knows roughly when to expect the van and if it's late, they generally phone 'up the line' to find out what's going on. Pretty much everyone knows exactly where the van is. Delays are usually down to someone struggling with the extensive paperwork, during which the sealed box is in full view of the waiting driver, the P.O. and Poll Clerk(s). Once all the boxes are picked up, the van races to the count. The drivers try to get there as fast as poss, so as to get the boxes droppped off and go home. We've been working all day by this point so all keen to get home asap. The boxes are then opened in front of lots of people. Where in that process is someone going to get time to both rub out and redo the votes? And don't you think the counters might notice rubbed out x's. You can't even rub it out in the polling station - if you want to change your vote we have to declare your original spoiled and give you a new ballot paper.
Can you tell I'm a bit fed up with people questioning me and my colleagues' integrity??? Last year, some of the vans were followed by people in cars who genuinely believed that the van pulled into a lay by for an hour or two, snipped open the boxes and changed all the votes. It's just not possible!