Bathroom scale will give you a good idea.
Fish scale... I got one from Stren and found it's accuracy way off and inconsistent. Mine was 12oz heavy at 9lbs.
Baby scale? Good idea!

I went to the office supply store and bought a 10lb digital postal scale. It was $85 but at least now I know exactly what things weigh. The 'wife' uses it to split up ground meat into 1lb freezer bags!
The 5lb scale was only around $45...

I have found that if you're building from a kit and know how to build light you should end up in the lower half of the weight range. If you've put together an ARF expect to be in the upper half of the weight range.