There is a florescent dye you put in the oil and use a black light to find it. However you should degrease and engine first. Check everything is snug, the valve covers, oil return line clamps, accessory clamps... Did you spill oil filling it? Sometimes that ends up in the bottom of the cowl.
A ground run may be in order first. Put cowl on to keep oil from being blow around too much. Try to get engine to full operating temp.
If you don't see leakage from dye indicator after a ground run or three, go fly, short flight or pattern with engine up to operating Temp. De-cowl and look for dye trace. If you go fly oil may blow around and make source hard to find. Clean engine and cowl as suggested above before looking for leak. A small leak can make a big mess over time.
Oil can come from many places, push rod tubes, rocker cover, oil return lines, main front seal, accessory case, accessories, oil pan, crankcase halves. Leak does not have to be right over airbox. Oil clings to engine case and flows around. Air flow under cowl in flight can be a tornado in many directions causing oil to migrate away from source in many directions. Typically airflow and gravity causes flow to be down. After that it can be a challenge to pin point.
Then what? If you find a small leak it may be best to leave alone. Many engines leak. Wing tanks leak fuel on large aircraft. There is an acceptable leakage. Put a pan under engine when parked and look how much is hitting the floor. Is there an oil film on belly or is it dripping off. I took my vacuum pump off and put a DIY blocking plate. It was not sufficiently sealed. One flight oil was dripping off the belly. I lost at list 1/2 qt. in short order. Reason for short flight test after maintenance. Have you removed and installed anything recently?
Lycoming, if no oil is leaking out of it there is no oil in it. (Paraphrase radial engine saying, same applies to old British Sports Cars and Motorcycles. Some may argue they are not leaking. They are simply making their territory.)