How can I check if there was a lot of interference on wireless channel?

What you get from radiotap, PPI, or other radio metadata headers is information that the Wi-Fi adapter reports. I don't think they explicitly report "hey, there's a lot of interference"; you might be able to infer it from the rate of CRC errors, but CRC errors aren't necessarily due to interference, and I'm not sure that interference would always cause CRC errors instead of, for example, a failure to start receiving a packet.

This article in Network Computing speaks of spectrum analyzers as being useful tools for detecting interference; it also says

A common question regarding spectrum analysis is why can’t the built-in, or USB, WiFi adapter be used for spectrum analysis?

It boils down to the limitations of WiFi adapters; they’re unable to demodulate non-WiFi signals.

When a WiFi device wants to communicate, it will perform clear channel assessment (CCA) to determine whether the shared medium is busy -- basically, seeing if any RF energy is detected. The kicker is that WiFi adapters can only hear WiFi energy, meaning other WiFi devices such as laptops, tablets, mobile phones, and WiFi cameras. Built-in WiFi cards cannot understand microwave signals or cordless phones transmitting on 2.4 GHz.