There is a relatively simple way to do good performance checks of systems without connecting gauges, but you need to be able to measure the airflow through the indoor unit.
Checking what is coming out of the registers with a flow hood doesn't count, and is often very inaccurate anyway.
Testo has a lot of details on it in their AC/R Application Guide.
They took down the page where it could be downloaded off their site, but I have it stashed on mine.
I hadn't previously linked to it on my site because Testo had people filling out a form before downloading it. Now that the form and download link are gone off Testo's site, I'll keep it on mine. At least until I see it available for download from Testo again, or someone from Testo asks me to remove it.