As far as I know, there are still no established criteria for separating Pale-legged (Ph. tenellipes) and Sakhalin Leaf Warblers (Ph. borealoides) in the field (at least, unless they are singing!). Plumage seems to be almost identical, and calls seem to be inseparable. I think that Mark Brazil lists some features in Birds of East Asia, but I;m not sure how reliable these are, especially outside the breeding season. Primary projection may be useful (Sakhalin is slightly longer-winged, and should have more primaries visible), but there is overlap between the two species.
Based on birds trapped and measured, Sakhalin appears to be a slightly later migrant that Pale-legged. As Mike mentions, individuals of the species pair winter in most years - the only birds trapped in winter have turned out to be Pale-legged, but there have been very few of these (four individuals) so it's possible that Sakhalin also overwinters.
Based on the apparently shortish primary projection (although not very clear on these photos) and the time of year, it seems that there is a good chance that this is Pale-legged, but I'm not sure I would want to say that for certain!
