pH test strips (litmus paper) would likely be the easiest (to find) however I've not had experience using them with soil. In the past I've used them for liquids (water solutions) and its very difficult to get accurate readings for a number of reasons... sometimes its difficult to get any reading using those paper strips... get them anywhere they sell aquarium supplies.

However I am more experienced with using reagent drop type pH tests (along with the other types such as ammonia, nitrites, nitrates, hardness, alkalinity, phosphates, etc. used for my outdoor ponds)... in fact I just used one a few days ago to do a soil sample of my bins (my first time for doing a soil sample). You'll have to get the test kit at a pet store or online (http://www.amazon.com/API-Freshwater-Test-Kit-tests/dp/B000255NAK/ref=sr_1_29?ie=UTF8&qid=1452178607&sr=8-29&keywords=amazon+pH+test+kit)... costs about the same as test strips... around $5. You start with a cup of distilled water (steamed or osmosis), put in samples of bedding/castings taken from several locations around the bin and stir... wait a minute or two for the solids to settle... fill the little test tube to the 5ml mark, add the reagent drops, put the cap on the test tube and shake for several seconds and wait a minute and compare to the color chart to read the pH. The amount of reagent in a pH test kit will do probably a couple hundred tests and the reagent has a long shelf life (couple years or more). If you are not color-blind in the region of colors associated with your particular pH test (which colors will depend upon the reagent used... as there is also a high-range pH test kit which uses a different reagent) it should be accurate to the nearest .2 pH.

Another way to measure pH is using an instrument. There are cheapy ones in the garden centers but they are notoriously inaccurate... if the stories on internet reviews are taken into account these meters cannot be trusted at all... their readings are typically all over the place. There are more accurate meters available from reputable companies such as Hanna but these are expensive ($80 and up) and must be calibrated often using special solutions (purchased separately) and still many have issues with probes needing replacement after only a year... its a very expensive proposition (however, typically the guys using these meters require some serious accuracy).