If you are taking 'em in some tap water, ask 'em what the ph of your water is.
Unless you take a sample of your runoff water, (water that comes out of the bottom of the pot after watering) they can't tell ya that one. If you take some in, they can usually test it for ya tho.
A freshwater aquarium ph test kit works for tap water, or, there's all sorts of ph pens.
Aquarium test kits are good because they are cheap, but you can only use them for clear water. (it's color-coded results get thrown off by additional colors being added)
Another prefered method of testing, is to get a ph pen. Below is a link to the cheapest one I could find, and it's working good so far....$60.00 plus shipping.
Discount Hydroponics - Milwaukee Testing Pens