A first-aid box is only useful if it holds what the first aider expects to find. Open it once a month, with the list from your needs assessment beside you (sample note).
Replace anything used or past its date: dressings, bandages, gloves, eye wash. Make sure the scissors and safety pins are still there.
Tablets and medicines are usually kept out of a workplace first-aid box; follow your own policy and local guidance. Write the date of the check on the inside of the lid.