First of all test flexor carpi ulnaris by getting the patient to flex the wrist and ulnar deviate simultaneously, showing this is weak.
Then test the small muscles of the hand by getting the patient to abduct the fingers and asking them to resist you pushing them in and doing the reverse with adduction.
Then do Froments test (testing adductor pollicis)
Then test flexor digitorum profundus ulnar innervation by asking the patient to interlock his fingers (as if they were praying) and note the inability to flex the 4 and 5th digits.
Then test sensation remembering that a lack of sensation proximal to the wrist indicates a C8 lesion rather than ulnar nerve.
Also remember that the ulnar nerve has a deep motor branch that supplies the hand, so if there are no sensory changes this may be the culprit.
Attempt to localise the lesion for example, if the lesion is at the elbow it will also involve the deep flexor of the little finger which will be seen on by protusion of the right 5th digit on interlocking
First of all test the anterior interosseos branch which supplies flexor digitorum profundus, flexor pollicis longus and pronator quadratus by holding the proximal phalanx of the thumb and getting the patient to flex the distal phalanx. Then do the pen touching test testing abductor pollicis brevis. Then test flexor digitorum superficialis by doing Oscners clasping test and making a fist (Benediction sign) noting the poor flexion of the 2nd and 3rd digits. Finally, make an “O” with the thumb and 2nd finger, it should flatten as FDP and FPL are weak.
Then test sensation.
Attempt to localise the lesion, for example, if there is no Benediction sign but there is weakness of APB, then the lesion must be between the wrist and forearm
As an aside( relevant to both nerves above), to demonstrate weakness of FDS of fingers 3,4 or 5 immobilise the other fingers by hyperextending them and the resulting flexion of the tested digit is carried out by FDS. To test the 2nd finger, it is necessary to do oppose the thumb and 2nd finger and demonstrate extension at the PIP joint.
Test all the muscles supplied in descending order i.e. triceps, brachioradialis, wrist extensors, finger extensors. Remember the posterior interosseos branch which supplies the deep extensors only, hence there is weakness of finger extension with radial deviation of the wrist on attempted extension with no sensory loss and normal reflexes.
Then test sensation over the anatomical snuffbox.
In a posterior interosseos nerve lesion or C5,6 root lesion there will be radial deviation of the hand due to relative preservation of ECR (supplied by C5,6 and radial NN) on attempted extension and weakenss of ECU.
Remember that finger abduction in radial nerver lesion my appear to be weak because of difficulty of spreading the fingers when they cannot be straightened. To overcome this rest the hand in the prone position on a flat surface.