The only 'real' way to know if it is working is to have had bloodwork done before you start and then again a couple months later...if not there can be certain signs of improvement such as less exhaustion, hair no longer falling out, better sleep patterns (you fall and stay asleep for example if you couldn't beforehand), better skin colour or 'clearer eyes', less brittle nails...they are all mild and very, very subjective but my personal rule is if I am feeling generally, all round better...
I have to climb a lot of hills and stairs in my environment and walk miles so if I am quickly breathless, have to hold the hand rail from the start or am sweating profusely when climbing etc then it is a sign for me that I am anemic or deficient in something (B's for example)...after a month or so on the vitamins/minerals if I feel 'stronger', climb faster or make it to the top without toppling over then I figure that they are doing their job.
I do get regular bloodwork done so that is a reasonably good indication.
I had a full body scan for vit/minerals using some sort of new-fangled-machine (not at all sure how scientific it is because it is being used in a mall) but they check you out for just about everything on the pannel and give you charts and percentage reports afterwards...(they are trying to sell you their supplements)...I had pretty much a perfect score and needed nothing so I guess that that is a good indication of their honesty...if you have a large mall near you, you can see if they can do your numbers and get the results that way. (it is free).
Good luck on your upcoming surgery...
Sharon