If you look in the fine print on the ticket, it will show the voucher applied. I'm looking at one of my JAX tickets, and it says "TVL CERT 150-NONREF" up in the cneter section where they list all the various taxes, fare codes, and routings.

In the bottom left corner, it does show the normal fare and full cost, but one glance at the fine print above will clearly show the use of a voucher.