I don't trust mine to be accurate on a day to day level, or even an hour to hour level when it comes to the body fat readings. However it is good at showing how I am trending over the weeks/months.
As far as weight, I would just test it with a dumbbell. On my scale, a 20lb dumbbell is exactly 20.0 lbs. For body fat percentage, I don't know.
Also I'm sure you're already doing this, but try to be consistent on when you weigh yourself since your weight fluctuates pretty wildly throughout the day. For example: first thing when you wake up in the morning, right after you pee.
Even body composition measuring equipment like DEXA or Body Pod do not give super accurate results of how much bone/muscle/fat you have (They are however more accurate than bioelectric impedance). They're an estimate at best and the readings can vary widely based upon water retention etc.
The key for bioelectric impedance scales is to measure at the same time of day, with relatively similar hydration levels and you can get a good trend line, but won't be getting an accurate representation of actual composition. In other words, wake up, go to the bathroom and then weigh yourself.
I trust weight on a relative basis. I was leery of the body fat. But I've had bod pods in the past. I generally have an idea that at X weight I'm Y percent body fat. The scale seems to track the correlation much better than I expected.
That said, I have too much Y. Need to lose some X.