Revoice Pro 5 shows a "Revoice pro" logo and an orange link button in the lowest left hand corner of the application window:
Revoice Pro 4 shows a "revoicepro4" logo and a neon green link button in the lowest left hand corner of the application window:
The exact version can be found in the settings window, accessed with the cog wheel button in the top right corner of the main Revoice Pro window.