Is there a way to tell/trick of the trade/ to see if a buried pipe is pitched properly?

I have a floor drain in my basement that i want to convert to use for the washing machine discharge (it's cast-iron pipe, the bell trap came out cleanly so going to use a fernco doughnut to a 3" ABS and go from there.

my only concern is that when i dump a few buckets of water down it, there seems to be a little bit that sits in the elbow (that i can see) i can hear the water drain away, but just want to make sure i'm not setting myself up for issues later on. (this will be correctly vented, etc)