"but as withholdings are reported on this"

Then there likely is some sort of sale, unless your taxpayer makes enough to cover the withholding and taxes out of ordinary compensation.

There might not be a 1099-B. That only happens if a broker gets involved. An inhouse sale means there is no broker, and a minimal or fractional sale to cover the additional tax and withholding is not unusual.

I guess the answer is: Look for evidence of all of the above. Ask your taxpayer for a statement of their current ESOP holdings and any transaction statements.