The position of a loon's legs isn't exacly an i.d. mark, unless you want to stretch it to, "If it's walking or standing, it's not a loon." Might help a neophyte with cormorants, I guess. This Red-throated Loon, under the toll bridge to Nummy Island, NJ last Saturday, shows why loons can't walk: their legs are positioned all the way at the back of the bird, exactly where you'd want them when your priority is swimming.
A Dunlin (rear) and Western Sandpiper (front) share space at Stone Harbor last weekend. Compare size and bills. Why isn't the Western a Semipalmated? The easy answer is because it is March, and the first Semis won't be around until late April. Plus the bill is out of spec, too long and drooped, and the neck too thick.