Step one... the sandpiper stands taller than the stint; check out the "upper leg" portion of each species:
In the above photo, look also at the primary projection; the stint has noticeable primary projection which gives the entire shape a slightly more tapered look.When next to the sandpiper, the stint has a slightly more pronounced mantle "V" when looking head-on:
In the next photo, you'll notice this particular stint has a darker head and more blurry, buffy coloring on the breast:
And in the above photo, yeah, just look at the bill length of the RNST compared to the WESA.