The voice synthesizer button is a 'split' switch - i.e. two separate buttons occupying the same space. The one that latches is the 'mute' button, and the one that doesn't is the 'speech' button.
Pressing the speech button at any time (with the ignition on) should either give you a voice announcement of one or more faults, or say "Welcome, the vehicle computer is now checking systems for you."
With the mute button latched in, the car will only produce a chime when an event occurs, and you have to press the speech button for the verbal version. With the mute button latched out, the car will speak to you automatically.
Apart from the obvious thing with the buttons, the only sensible test I can think of is to try to trigger a warning. If you've had the fuel into reserve yet, then that should be accompanied by a spoken "Warning, fuel level low." If you get a beep instead (and the mute button isn't pressed in) then the car doesn't have the voice system. If you get nothing, then the voice system may be faulty.
If you can't wait for low fuel, then try driving off with the handbrake on its lowest notch (just enough to light the lamp on the dash.) After driving for a few yards, the car should give a spoken warning. Similarly, if you drive away without your seatbelt on, the car will (from memory) beep for a short while, then give you a spoken warning. Later versions (without the voice synth) beep for a while, then give a more urgent (higher pitched and louder) beep, then finally shut up and leave you to take your chances
If the car shows all signs of having the voice system but behaves as though it hasn't (i.e. two stages of beep on the seatbelt warning) then it might be that the car has 'forgotten' that it has the feature fitted. A Renault diagnostic should prove the point, but it'll cost you