These are our two smallest shorebirds, and two of the most common: Least and Semipalmated. Most other common species are distinctly larger, with the exception of Baird's and White-rumped which have very long wings extending beyond the tail.
On these two excellent photos compare:
Bill shape: Distinctly curved and fine-tipped in Least, straight (sometimes slightly curved) and blunt in Semipalmated
Leg colour: yellow in Least, dark in Semipalmated
Overall colour: darker with some warm rufous tones in Least, pale with very little warm tones in Semipalmated
Breast: Somewhat distinct breastband of streaks on Least, limited streaks on Semipalmated
Posture: Least typically looks like it's "crouching", Semipalmated is typically more upright