To use the calendar test you simply add the days to maturity of your watermelon variety to the date you planted it. I'm usually unable to use the calendar test because I'm just not organized enough to keep track of when I first planted the seeds. I'll skip that one.
The shriveled tendril test says that your watermelon is ripe when the tendril closest to the fruit turns brown and shrivels. The trouble with this test is that it's not always reliable. Sometimes my tendril has shriveled while the fruit is still growing. The tendril on this particular fruit shriveled several weeks ago, but I didn't want to pick it too soon.
The fingernail test says that the rind of the melon should toughen and resist slightly when you push into it. My watermelon failed this test. My fingernail went right into the rind.
The most mystical test is the thump test. The thump test states that if you slap the fruit on a watermelon, a ripe fruit should sound different than an unripe one. I understand it has something to do with the water sound inside the rind. I call it mystical because I've never been sure exactly what it's supposed to sound like. I dutifully thumped my melon, but did it really sound just right or not? This leads me to my new indisputable test, the exploding watermelon test.
As I was thumping away at my watermelon I noticed something didn't sound exactly right. I thumped again and the fruit exploded in my hand! The whole thing cracked in the center 3/4 of the way around the melon. I guess it was ripe after all.