Can a doctor tell during a saline ultrasound if you have hydrosalpinx ?

Maybe : It depends on the technique and your anatomy. There are some catheters that instill a mixture of bubbles and saline into your uterus, so you can see the fluid moving through your tubes. But regular saline is not well visualized in the tubes. A hydrosalpinx can sometimes be seen on a regular ultrasound if the tube is distended enough. Having said all that, an hsg is probably the best test.