The line between winter and summer steelhead is not absolute. Generally, a summer run enters freshwater many months before spawning and sexually immature, whereas a winter run enters a few weeks to few months before spawning and is already starting to sexually mature (males have an elongated lower jaw even when chrome, gonads are relatively developed). They pretty much all spawn late January into July. Not sure where Spawnout got his info on Columbia river fish. (Deschutes fish arrive June through December and usually spawn in late winter, early spring. They're all considered summer steelhead, by the way.)
I once heard several biologists admit in a discussion of steelhead run timing (on the Sandy River), that they cannot tell in many cases when a given steelhead in a given river will spawn (based on appearance). Thus, it can be hard to tell.
If the fish is hatchery origin, sometimes you can tell by the finclips which strain it is.