How do I determine the ph of juice extract from a plant?

It is simple to get pH paper at any drug store. If you are doing a lot of tests you can use a pH meter: www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=pH+meter

If you want to be cute, you can use the juice of boiled red cabbage. If you want to be clever, there are dozens of dyes that change color at certain pH. For instance, college students love to give someone a beer with a dye that turns urine some odd color.

You should get a chemistry book and study the subject because there is a lot more involved than just the equipment.