My partner says i may have depression. how do i tell the difference between depression and just having a bad day?

Degree of "bad day": For this you'd have to review the intensity and degree of "bad day." if this involves only occasional crankiness, it's probably just a bad day. If it's 2 weeks or more of sad/irritable mood, sleep disturbance, crying spells, appetite and energy changes, and suicidal thoughts, it's more likely to be depression. See your physician for an initial workup; psychiatrist consult may be useful also.