> How could I go about proving it to myself...
Much the same way Devs store projects on GitHub-- start building up your portfolio of talent profiles and notes on individuals that you interview & assess. It's a practiced art-- you become exponentially better the more people you evaluate. Also, understand that people do change and grow as they progress throughout their careers. Take the long view and be sure to follow up periodically.
The best executive leaders create a talent pipeline, typically meet & informally interview 2-3 people a month. Incidentally, this builds-up an amazing network of true contacts, which is to say NOT Linkedin contacts. The best primer on this process is Bradford Smart's Topgrading > http://www.goodreads.com/book/show/915182.Topgrading
> then selling this service if I am capable?
Connecting with buyers, in this case Hiring Executives falls into the specialty function of Sales & Business Development. Professional Services firms are always chomping at the bit for rainmakers. If you're serious about this path, take a development course like Miller Heiman > https://www.mhiglobal.com/