Structure is often useful in stint ID. Long-toed is even smaller than red-necked/little (useful in comparison). It also tends to seem very small-headed and thin-necked, and more upright than the other species. The legs (when visible) are shorter than on other stints. The bill is slightly decurved.
In terms of plumage, Long-toed in spring is very bright with orange tones (not brick-red as on red-necked). These often seem a bit more intense than on Little Stint, especially on the face and breast - Little would show a pure white throat and centre to the breast whereas Long-taoed has a complete breast band. Overall, I think that Long-toed often gives the impression of a tiny version of Sharp-tailed Sandpiper. In winter plumage, Long-toed appears browner, less grey than Red-necked/Little. Leg colour is also very useful, of course (pale on Long-toed, dark on red-necked/little)
Habitat is also useful - Long-toed tends to occur more on freshwater habitats (fishponds). At the moment they are often outside the boardwalk hide but favour the more vegetated areas or areas with algae on the surface of the mud. Red-necked/Little tend to occur more on the open mud, usually well away from vegetation.
