The scale LIES, but the measuring tape NEVER LIES!
The reason I say that the scale lies, is that everyone, especially menstruating women, tend to carry a certain amount of fluids and bodily waste.
People can carry several pounds of bodily waste in thier intestines at any given time which will look like weight gain on the scale.
People can also lose bodily waste in one bathroom visit which will look like weight loss on the scale.
The only way to know if you've lost fat is by the use of a measuring tape, and/or calipers that measure skin folds.
Weight is only useful for one thing and that is to determine the amount of calories you need so that the amount of calories you must cut from your caloric need to maintain weight can be calculated. For this reason, people should measure everytime they weigh in order to make sure thier loss is fat and not bodily waste.
Weigh atleast once per week.
People who are part of the National Weight Loss Registry who have maintained thier weight loss for several years weigh themselves every single day.
For an accurate weight, weigh yourself first thing in the morning after emptying bodily waste, before eating or drinking anything, and weigh yourself while while you are nude. Keep a written record of your weight.
When measuring, make sure you measure in the same place everytime, and keep a chart of your measurements.
Areas to measure:
Neck
Chest
Upper Arms
Waist - the smallest part of the torso
Lower Abs - the largest part between the belly button and the pubic area
Hips - the largest part of the buttocks
Upper thighs
Calves