In what way do you think the voice has been altered? There are some ways you might be able to find inconsistencies. If you are looking for chopped up audio you can usually tell this by running the audio through a sonograph but unless you know what you are looking for the process can still be a bit mysterious.
I have some experience with sound engineering. You can't always be sure if audio has been modified. You can maybe see little "tells" that would perhaps indicate editing.
If you want to know if a voice has been altered in some way though, this would require lots of reference material that you can be sure has not been altered in any way.
For example, if you think the pitch of a voice has been altered, this will modify the formants. When a human speaks, the mouth acts as a filter. If you look at this in a spectrum analyser you can see the same shape moving up and down the range of harmonics, but if it has been pitch shifted then this shape will spread out or stretch the higher it's pitched. It's harmonic composition will be in a different ratio. Some people call this the "Pinkey and Perky" effect but I've also heard it called "chipmunking".
I guess I would say, it's not an exact science and prone to error, particularly if you don't have a frame of reference for the pure unmodified voice.