I agree with birdseye, these are very hard to tell apart, especially in bad light. The males, though, when seen in direct light, have black band and then a purple one, while the Ruby-throated has an all red throat. Also, the Ruby-throated holds its tail still while hovering, and the Black-chinned constantly pumps its tail, so this might be a useful clue. More pictures would help, too, though it will still be very hard to identify the female.