Final answer:
A nurse preparing a sterile field knows that the field has been contaminated when sterile items are wet and the nurse uses sterile forceps.
Step-by-step explanation:
A nurse preparing a sterile field knows that the field has been contaminated when sterile items are wet and the nurse uses sterile forceps. In a sterile field, all items and surfaces should be dry to maintain sterility. If any item or surface becomes wet, it indicates that it has been contaminated and cannot be considered sterile anymore. Additionally, using sterile forceps ensures that no contamination is introduced to the sterile field.