How can you confirm a soldiers MOS?

You should know your MOS when you enlist. If you enlisted prior to the reorganisation which occurred between 2001 - 2004, I could understand some confusion regarding one's own MOS, but you should have received orders assigning you your new MOS, the same way you received orders assigning an MOS to you when you completed AIT. If nothing else, ask to take a look at your 201 file - it'll be in there.