Double-hung sash slides back down before it’s fully open

What’s the safest way to narrow down why a double-hung sash won’t stay raised? A failed balance seems possible, but I’m not sure how to tell that from a pivot bar that’s come loose or a sash that’s binding.

Does it make a difference whether the sash drops on one side or both, or whether it feels unusually heavy and moves unevenly? I’d also like to know what can be checked around the balance shoes and pivot bars without removing the sash. If it won’t stay supported, should the sash be held in place and left alone until a window repair professional can assess it?