Find out what the major nectar sources are in your area. Mine are:
Maple - Rapid increase in brood production – Begins in late Feb/early March and lasts several weeks as different varieties bloom at slightly different times. Weather is often fair enough for inspections during the maple bloom, and hive conditions may indicate that it is time to reverse brood chambers.
Dandelion - time to begin adding supers, reversing brood boxes, and other swarming counter-measures– mid/late March. Main Swarm issue usually begins about 3 weeks later.
Apple/fruit trees - Start of swarms issuing – April
Poplar, Black Locust - Main nectar flows in mid TN – May
Goldenrod - Begin getting colonies ready for winter – September
There are minor ones too of course, but in my area those are the important ones. Yours will surely be different.
Watch for those flowers to appear, and when you see nectar coming into your hives - that is really the main thing. There could be a main bloom going - poplar for example makes or breaks my honey crop - but if the weather doesn't allow the bees to go out and get it, or if excessive rain washes it out of the flowers (that happens) then it won't really be a flow. Nectar in the hives is all that counts.