On instruments such as guitar and piano I find it is easier to measure progress. You have objective devices such as the metronome and generally when you get things wrong the sound is very dissonant. With voice, I'm finding it much more difficult to measure progress. I practice breathing, scales and songs every day but I'm not sure how I can measure improvement if any. Are there any tools or techniques to determine if I'm getting better with practice?
There are a range of programs which can help you perfect the pitch of your singing - one that is surprisingly good (i.e. it actually works) is the Rock Band game on Playstation/XBox etc.
When set on Expert, the tolerance for getting the correct note is quite tight, and of course - it is fun. Over time you should expect to see your score increase.
Tuning apps on your phone can be useful - I have the DATuner on my Android, which I use for tuning my guitar if I am out camping, but it also helps me zero in on pitch when trying to sing in tune. It doesn't have any functionality for measuring improvement over time, though.