I don't think there's always a clear-cut answer, but I do find that mood charting and journaling are helpful to figure out the difference, at least after the fact.

I know that stress (especially medium/long term) can make me unstable and cause episodes. What causes stress will differ from person to person. The difference between a stress-induced bipolar episode and a situational mood is that with a stress-induced episode I generally wouldn't be able to tell you in advance which way I'll go, up or down. The nature of the episode can be the opposite of what you would "logically" expect.

Sometimes it's a combination. It may start out as a situational mood and then the stress and/or lack of sleep from whatever happened causes me to spiral further out of control and the situational mood can transition into a bipolar episode. Sometimes I start out having an episode and something situational happens to make it worse.

For me, I would associate a situational depression with more conventional feelings of sadness, which is different from what I feel in a bipolar depression, although again, sometimes the lines get blurred and it can get complicated to figure things out.