I know this has probably been covered, but I would like to know what test you find is easiest. I am not geting any younger and staying up nights is getting harder, so it seems a lot of people have good luck with the testing, time for me to give it a try. Would love to know what products seem to be easy and accurate.
I use some aquarium strips and a jug of distilled water I get from Walmart (or order in bulk online) - strips are called called "QuickDip" by Jungle Labs and it's a 5 in 1 test but you only need to watch calcium (hardness) and most importantly pH. They are $12 for 25 and what $2 for distilled water, so very economical! Any spa or pool strip will work as long as it has a range of up to say 300 or more for calcium and at LEAST 6.2 or lower on pH. 6.5 or 6.8 is not low enough, and it can be challenging to locate those that go lower I've found. Mix 6:1 - i.e. 6cc water and 1cc milk, mix gently and dip!
We both have full time careers so foaling out 4-6 mares a year and trying not to go to work a zombie or calling out of work for what ends up to be nothing has to be minimal - and last year we had 5 foals and didn't miss hardly any sleep until a few hours the night each was born.
Once you see a true 6.2pH (not 6.4, we've found) or lower, you should have a baby that night or the next (within 36 hr usually). The calcium should be 250-300ppm or more at this point, and like EllenAspen said, the taste and looks are key too. Also it will be sticky, sticky, sticky!!
Of course any mare can do anything at any time but we've gotten to where we strongly rely on the pH testing probably almost too much! This has worked even for our maiden mare a couple years ago. She did stay at 6.2pH for 3 days and then when she finally did foal the foal was dysmature and a dummy foal so when they hang at "ready" too long it makes me nervous. (not to scare you, sorry!)
And one more thing - I have heard and found reasonably true that you can check their temperature daily in the evening, and when their temp is significantly lower than normal (say .5-.8 degrees or more) they should foal within 24 hours. It has been true for the mares I have done this with but I am lazy now and just do the milk anymore.