It's easy to forget if you took a pill or multivitamin, particularly if you do it in the morning before your brain completely wakes up. Luckily, there's an easy habit you can use to make sure you never miss a dose, or double up.
Once you take your pill, flip the bottle upside down. That way, you'll always know which pills you've taken, and which you haven't. Before you go to bed in the evening, flip the bottles right side up again to reset the system.
Obviously, this isn't foolproof, since you still have to remember to flip the bottles twice a day. But if you can get into the habit and stick to it, you'll spare yourself a lot of hand-wringing.