Isn't it on the W-2?

it doesn't appear to be.  The employer wouldn't know that the employee sold the stock in 2023

Why wouldn't they? The administrator and the employer usually communicate.

There's nothing in Box 14 of the W-2?

I'd ask the client for the pay stubs covering the sale date(s).

I've seen it both ways, but if it's the current employer it's usually in the W-2.  I get year-end paystubs from all of my clients with equity compensation.