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.
Getting your pitch right as a singer is about as important as setting your foot where you intended as a dancer.
I would not really take this as a metric of vocal progress of a "singer" even though there are successful singers astonishingly off-pitch (particularly noticeable with long notes). Being off-pitch makes it unfeasible to sing leggiero lines, coloratura etc. For example, try doing Bobby McFerrin's part (he's doing the arpeggiated chords) inand see how surprisingly hard it is to get each note right when you don't "slide" together a whole melody line but sing isolated short notes. If you can actually do that in a pleasant manner, you already have a lot of control.
Some more subtle measures are how good you are able to connect your melody lines, how good you are arranging your breath so that you don't need to interrupt phrases, how quiet you are able to sing at your higher range for how long before you tense up and so on.
It makes sense to retain a few basic practice songs for a long time (like years and decades) and see how you can improve them when not having to focus on the notes/text/rhythm, and how you can bring fresh elements into them.