I agree that it is better to get what you need from your diet and would disagree that it doesn't do any harm to take more just in case. The extra you take have to metabolised and go through the liver, exceretd by the kidneys and excesses of some can do harm.
Don't worry about that depletion stuff - I mean it's twaddle! LIVING depletes vitamins and minerals and different metabloic activities utilise different vitamins and minerals. Full stop!! That's why we eat food! You just need to make sure your diet is very good with a few well chosen supplements as honeybun and Dancgingirl says.
Personally I do not take a mulit-vitamin because it gives you the RDA of everything which you don't need with a good diet. Maybe some Vit D in late winter - but if you get out in the sun without sunscreen at this time of year you shouldn't need it - it's stored in the liver and found in lots of foods. Maybe fish/omega oils if you diet doesn't contain enough of these. Maybe a herbal iron tonic (I take Floradix now and again) if you are bleeding regularly.
The supplements I do take I only take now and again and way way less than what they say on the label - so I use them as an occasional top-up so as not to overload my system, but make sure my diet is excellent and go out in the sun.
I agree don't worry about getting tested - just look at your diet and what's in it. Look at the NHS website for info. Lots of whole grains, nuts, beans, pulses, wholegrain cereals, olive oil, fish, chicken, milk products, masses of fresh fruit and veg, few processed foods, cook from scratch - and you can't go wrong!
Hurdity x