"How can you tell when a tree's bloom time will be?"

Decide what type of tree you're interested in; apple, pear, peach, plum ... and search for bloom-time charts. Keeping in mind, actual bloom dates in California will be earlier than Washington. I suspect most "Pollination" charts take bloom-time into consideration.

Most fruit tree bloom-times are listed in order of earliest to latest. Good cross pollination must include comparable bloom-times. I've found it critical in both apples and cherries. A Gravenstein apple is a very early bloomer (it's also a triploid and perhaps a bad "example" because it needs two pollen sources) ...and though many apples could pollinate it, only those blooming at the same time (like Transparent and Summer Red) will actually work.

Also, check here for apples (scroll down to "Ted"): viewtopic.php?t=312

Good question, and often overlooked.