speedtest app to see what speed you're getting.
Wifi Analyzer to see how congested the channels are.
It's worth trying out the Wifi Analyzer app all around your flat to see which is the best channel to use on average across the flat.
If there's a lot of traffic in similar channels it can really introduce noise and drop the speed.
I personally think one of the things that's lacking in modern wifi is some sort of automated tool to tune the reach/spread within congested areas.
Back when I was running Tomato open source firmware on an old router/AP, I was able to tune the wifi so it only reached the limits of our flat, but the signal was much stronger as a result.