This is a difficult one, especially for people who aren't suffering considerable illness.

For myself, I do notice the effects of supplements especially when I was quite ill with digestive issues.

I noticed minerals helped when I had panic attacks and anxiety. I had tried various tablet forms but it wasn't until I got a liquid mineral supplement that I noticed anything different. The liquid minerals worked quite quickly and within a few days I was more calm.

With vitamin D, I notice healing from pulled muscles is much quicker. I think vitamin D took the longest to see results though at a few weeks.

I notice with cod liver oil that my skin is not as dry and I heal faster from cuts and sores. This was almost an immediate effect, within about a week. Whenever I stopped it I noticed more skin problems which would go away when I took it again. Didn't help clear up acne though.

With each of these supplements whenever I would stop them I would notice within a few weeks that I wasn't doing as well. That said, now that I'm feeling much better, I can do without the mineral supplement altogether and don't notice much effect if I take it. The vitamin D I continued taking during the winter. The cod liver oil I take only occasionally.

I would not recommend taking Calcium without other minerals. If you are healthy and eating a good varied paleo diet, I would think the only thing you might need is vitamin D. I find the best vitamin D supplement is one that is dissolved in oil in a gel capsule. If you are eating industrial meat, then maybe a fish oil as well for Omega 3 balance.

If you're already on a paleo diet, and you're generally healthy, you will probably not feel much different on or off supplements. You may notice a difference if you take supplements for a specific purpose, such as minerals for muscle cramps, or cod liver oil for softer skin. Vitamin D does not make me feel any different, but my dentist and hygienist commented that my teeth looked better on my last checkup.

Some supplement effects can be measured either directly or indirectly by improved lab values. Iodine supplements may improve thyroid function. Magnesium supplements will increase magnesium levels. Iron supplements will increase hemoglobin. Fish oil and niacin lower LDL and C-reactive protein, while raising HDL.

Finally, you may want to consider getting your 25-OH vitamin D level checked. You may not feel any worse with a low vitamin D level, but it may make you more susceptible to certain cancers (breast, colon) and respiratory infections (influenza, rhinovirus). Get your 25-OH-D level up to at least 50 ng/ml ( http://freetheanimal.com/2009/03/vitamin-supplements-part-four.html ). Richard Nikoley has extensive posts about vitamin D in the Free the Animal archives.

I have a very high anxiety level naturally and see a big difference when I take l-theanine. I think the brands matter too. The one Im taking now is Enzymatic and I notice a much bigger difference than other brands (but its also gluten free so maybe thats why). I also take SAM-E for mood and it is a noticeable difference. Others such as fish oil, probiotics, and a multivitamin are more subtle. I guess if you have a present malady you will notice a difference if the supplement is meant to address it but if its for long term health it is much more subtle.