Final answer:
The nurse should validate the laboring client's allergy history before giving butorphanol tartrate (stadol).
Step-by-step explanation:
The most important assessment information for the nurse to validate with the laboring client before giving butorphanol tartrate (stadol) is the allergy history. This is crucial because the client may have a known allergy to the medication, which could result in an adverse reaction.
While the pain level is important to assess, it may not be the most critical information to validate before administration. The fetal heart rate is also important to monitor, but it may not affect the administration of butorphanol tartrate directly.