Weigh yourself, then weigh yourself holding the bag, the differance is how much the bag weighs.
IME, unless the bag is carry-on size, or a long, skinny duffel that you are holding vertically against your body, this method can easily be off by over 5 lbs.
For a bathroom scale to work right, the weight has to be centered over it. A large rectangular pullman will usually register badly in your arms for the same reason it would if you put it directly on the scale -- too much of the weight is off to the side of the scale when you hold it, unless you hold it vertically over your head (and how many of us can do THAT with a 29" pullman that is fully packed?)
I recommend putting an empty cloth duffel into your largest checked bag, so that you have a way to take off some slack and lower the weight. Of course, in doing that you have created an add'l checked bag, so you don't want to start out with the maximum number of bags. (On most airlines, the penalty for an extra bag is less expensive than for an overweight bag, but don't take my word for it -- check your own airline. Either way it is money you don't want to spend.) Bags that exceed maximum size tend to be REALLY expensive, so measure your largest bag carefully, grab handle included, and if it is a softside, don't forget to make allowance for at least 3" of "bulge" factor on exterior pockets.
For situations where I really want to know the exact weight before heading to the airport (overseas flights where I'm taking things to family, etc.), I take luggage to my veterinarian's office, where they very kindly let me use the floor scale. BTW, be aware that most large wheeled suitcases weigh more than 10 lbs. when empty, some are nearly 20 lbs.