Our HTTP inspection tool of choice is of course Fiddler, however if you open this while using QGIS to connect to a web service, you see… nothing. So you need to tell QGIS to pass requests through a proxy, using Host = localhost and Port 8888 as shown below:

QGIS menu Settings > Options > Network:

Requests now start appearing in Fiddler.

However, there’s a small gotcha… once set into this mode, requests fail entirely if Fiddler is not running.  A WMS will simply fail to draw, and connecting to a WMS server gives this error:

Could not understand the response. The wms provider said:

Download of capabilities failed: Connection refused

This cost me half an hour this morning, having forgotten that I had set QGIS into this mode a few days ago, grrr.  Here we are after I’d remembered the solution, showing a WMS of archaeological sites recorded by the National Trust, in Snowdonia, Wales.