Stoats are stoatally different and weasels are weasely distinguishable.
Stoats are bigger - but a big weasel can be as big as a small stoat.
And female stoats are smaller, so it terms of size, they are easily confused with weasels.
Stoats have a black tip to their tail - but you will be lucky to see it when the animal crosses the road or track in front of you.
Stoats sometimes turn all white in winter, whereas weasels do not - but that is not going to help you distinguish them in summer.
So How? Stoats are slinky and bounce along. Weasels zip along like little rockets.