Questionnaire/history:

Painful thumb after forceful abduction or extension (?got caught on something or hit by a ball)?

(Note: ulnar collateral ligament (UCL) vastly more frequently injured than the radial CL)

Examination:

(Note: Assess the non-injured side first, grip the metacarpal in one hand and the proximal phalanx in the other and try to distract the phalanx away from the index finger with it in both a straight and flexed position)

Increased laxity (prompting treatment as a tear)? 

Management:

Same-day X-ray (?avulsion fracture) if available (but not essential as can be done in clinic)

Suspected UCL injuries: as a minimum thumb immobilisation (usually in a thumb spica which allows movement of the other digits, either using a splint or a cast) and referral to fracture clinic