Hydrometers
When sealed, methanol has a shelf life of ~2 years. Once the container is opened it will not last nearly as long because it will absorb water and no longer be pure methanol. If you are buying methanol from a much larger drum, there is no way to know the quality of what you are buying without a hydrometer to test it with. Even if you buy a sealed drum, once it is opened the quality will degrade over time as it absorbs water.
You can buy a hydrometer to test what you're using to ensure that your injection is consistent over time.
Here is a $40 hydrometer setup made for testing methanol. I honestly dont know the difference between this and some of the much more expensive options:
http://www.motorsportsracingfuels.ne...D=168&sent=yes
Here is a $48 hydrometer setup which sounds like it requires a 250mL hydrometer cylinder:
http://www.ridgecrest.ca.us/~hideseng/prod05.htm
Full kit is also listed here for $279:
http://www.ridgecrest.ca.us/~hideseng/prod03.htm
VP also has a kit listed here for $235:
http://www.vpracingfuels.com/hydrometer.html