Re: How would i measure the amount of rust?

I'm going to make the assumption that you actually want to measure 
rusting and corrosion. The difference is that "rust" is actually the 
common name of the chemical iron oxide, and not all the iron that goes 
away, especially in an acidic solution, will become rust.
The way I would measure the total corrosion is by mass lost. You'd need a 
bronze wire brush to scrub the nails before and after their "bath" -- you 
want to use bronze because it's softer than steel, and won't take off 
anything that hasn't been corroded.

A list of things you'd need:

5 steel nails
5 closable containers, big enough to hold the nails
the various liquids you want to test, enough to submerge the nails
a bronze wire brush
a scale that can measure in grams, preferably in tenths of a gram
Work gloves
Safety glasses/goggles
A lab apron, or some other waterproof way to cover the front of you while 
you're working.

First, scrub the nail with the bronze brush. You want to remove any 
surface dirt and corrosion. I strongly suggest using work gloves and 
safety glasses/goggles to avoid any injury. Wipe the nail clean with a 
clean, dry wipe (any clean rag or soft paper is OK) to remove any small 
dust or debris. Record the inital mass of the nail, then put it into you 
testing container. You want to completely cover the nail with the test 
fluid; leaving part of it exposed will only be "fair" if you can figure 
out how to leave exactly the same portion of each nail exposed. Do the 
same thing (clean, wipe, record mass, submerge) to each nail. Now just 
wait. You can choose to either agitate the containers (that will increase 
the speed of corrosion), or not; if you do, you have to make sure that 
you do exactly the same thing to each of them. I would suggest not 
agitating them for that reason. Wait until your test period is over, then 
remove each nail, and again wearing goggles/gloves/apron scrub the nail 
with the bronze brush down to bare steel. Be careful, as the liquid and 
rust may make the nails slippery. Again, wipe the nail off with a clean, 
dry wipe. Record the final mass of the nail. For each liquid, calculate 
(initial mass - final mass). Then multiply that number by 100, and divide 
by the initial mass. This will give you the loss as a percentage.

I would expect that the most acidic liquid will cause the greatest 
corrosion, although tap water and rainwater should be very close, and 
cola, 
vinegar, and orange juice should also be fairly close. 
This chart will show 
you why I'd expect that.

Good luck!