Well, you can't necessarily distinguish a unipolar (major) depressive episode from a bipolar depressive episode. If you have a manic episode, you're automatically bipolar.

However, bipolar depression is more likely to have "atypical" features.
- Oversleeping instead of insomnia
- Overeating instead of not eating
- Ability to be briefly cheered up by very positive things.
- Sensitivity to criticism

Plus, bipolars are more likely to have trouble getting up in the morning rather than waking up early.