How fast is my streaming?

[runf]

I guess this is a general networking question as well as a Mezzmo question.
Also please excuse me if this has already been answered and also my relative cluelessness.
I'm using a powerline adapter (Tp-link AV500). I also use wireless.
I'm streaming to a Nexus 9 (wireless ac) and a Sony KDL 46NX-700 TV (powerline).
Most movies (usually MKV), stream just fine but some want to transcode which usually hangs things up.
How can I tell how much data I can push from the computer to either my tablet wirelessly or to my TV via the powerline adapter?
I've heard that powerline adapters often don't reach advertised speeds, but how can I tell what the speed really is?
Hope I'm making sense.