"Testing" after MCs

I see posts in various rooms of women asking each other if they've had "testing done" to figure out why they miscarried. 
My doctor only did a few blood tests to make sure my HCG levels were decreasing. Was she negligent? 
So, what do you all mean by "testing"? What kind of tests did your doctor do, and what did s/he find out?
Thank you in advance for sharing your stories.