If you want to check if a WMS service is available you can test with a request from your browser.

Find out the address to the WMS service by right clicking the layer > Properties and then go to source:

Use parameters to alter the result:

LAYERS= specify the layer you want to view

SRS= specify the coordinate system

WIDTH/HEIGHT= number of pixels on your screen

BBOX= lower x, lower Y, upper X, upper Y coordinates

http://hades.slu.se/lm/topowebb/wms/v1?SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&FORMAT=image/png&TRANSPARENT=true&LAYERS=topowebbkartan_nedtonad&STYLES&SRS=EPSG:3006&WIDTH=768&HEIGHT=333&BBOX=405000,6400000,410000,6405000