Weighing would only work if all the rolls were uncirculated and guaranteed to have 50 US half dollars. The problem is, that is not the case. 40%ers have a similar weight to nickel clad. This creates a problem if any nickel clad is worn, or any 40%ers are worn. 90%ers are even more prone to wear. Some rolls are short. A short roll by one coin will be enough to miss silver. In addition, there are foreign coins which may be heavier or lighter, which alters the weight. Would weighing give you a general estimate? Yes, but it is too general to just cherrypick the rolls this way. Most unsearched roll sellers on eBay use this method, since it will mean they won't give away a massive silver roll, and the occasional 40% will be in them.
Weighing coins is very good and helpful, as others have said.