Any time your medication bar coding system fails, that should serve as a red flag for you. Go through the five rights of medication administration, and if you still feel unsure after that, call pharmacy.
Always stay in the room with your pt while they take their medicine. If you don't, then you have no idea whether they actually took it, dropped it in their bed covers, or threw it away. The only exception should be an absolutely emergency - i.e. you're giving a pt their medicine and you receive notice that your other pt is coding. In that case, find another nurse to stand there with them and make sure they take their medicine.
Pay extra special attention to your confused, elderly patients. It's so easy to miss warning signs for sepsis, pneumonia, CVA, MI, etc. with them. If they're not reacting or responding to you the same way they did 2 hours ago, be concerned. If they have a low grade temp, they're asking you for extra warm blankets, and they're NOT there for an infection, be concerned. Try not to miss the little things.
On the same note, pay extra attention to your patients in restraints (whether that's all 4 bed rails up, soft wrist restraints, 4-point leather restraints, or chemical restraint). Be sure to do ROM, to offer water, snacks, toileting, to check their skin for new areas of breakdown. It sounds so simple, but you'd be amazed just how many nurses "forget" about these patients.
Find your resources (more experienced nurses who appear to hold their own, your charge nurse, lab, pharmacy, RT, that doctor who doesn't mind to answer questions, whoever it may be) and build a rapport with them. You will likely learn more from them than you realize, and their help can be invaluable.
Oh, and listen to your gut instinct! Even if you're wrong, it's better to be extra careful than to be extra careless.