If the plant is growing near water in consistently moist soil, is tall (0.75–2m), has purple splotches on the main stem, and is heavily branched with small umbels of white flowers, it is probably poison hemlock and should be avoided.
Given the typical conditions under which it grows, its height, purple spots on the main stem, and profuse white blooms in a bushy arrangement, this plant is likely poison hemlock and should be avoided.