(At Kaiser, if the lab has the work order, you just go in any time and give them your card, and they do the test. It's pretty convenient, you don't need your doctor to see you beforehand.)
You could ask your doctor to send in a lab slip for an hCG test any time between now and then, or two hCG tests if the first one shows you still have some hCG in your system. She would then have the results in time for your appointment, and if the numbers are not going down, she would be ready to do an ultrasound.