How do doctors diagnose a blood clot even without surgery or internal examination?

Ultrasound: Noninvasive, painless, but how to see clot.

Ultrasound: Ultrasound is a very sensative test used to diagnose deep vein thrombosis (dvt). Conventional ultrasound uses painless sound waves higher than the human ear can detect that bounce off of blood vessels. A computer converts the sound waves into two-dimensional, black and white moving pictures called b-mode.