Next you could see if any of the draining areas have any evidence of infection, like tonsils, the skin, tooth abscess
or an ear infection. Infections like tuberculosis can cause lymph adenopathy. It would be worthwhile to see if other lymph groups are enlarged namely, axilla and groin, these can be palpated, the others like those in the chest or abdomen may need a CT to detect their enlargement. Also correlating with blood counts, could help with further analysis.