The 2nd through 5th fingers of the hand have two flexor tendons: the flexor digitorum superficialis (FDS) which attaches to the middle phalanx, and the flexor digitorum profundus (FDP) which attaches at the distal phalanx.
Testing the FDS:
Brace the other fingers in extension and ask the patient to flex at the PIP joint. The finger being tested must be isolated because the FDS muscle comes from the forearm. Bracing the other fingers neutralizes the FDS muscle proximally.
Testing the FDP:
Brace the finger at the middle phalanx and ask the patient to flex at the DIP joint. Remember that the 3rd-5th fingers share a muscle belly, so it is normal for the fingers to move together, even when trying to isolate just one finger.