SKY no signal being received - how to test dish & cable?

[trackdemon]

Sky box decided to throw up the "no signal, please switch off" message and after leaving it unplugged from mains for a while, it's still the same. Having had this before & replaced the box to resolve I sourced a replacement Sky box but it's exactly the same, so I figure it's a problem with the cable or dish? Does anyone know how I can test the cable in the house to see if it's getting a signal - is there a tool you can use for this?

Cable to dish is probably 30ft long, but it's wrapped in piping all the way through the garden so hoped that would protect adequately.

[trackdemon]

Thanks for replies chaps, I've tried a couple of other boxes and same symptom - no signal. So it's either cable or dish - but how do I test which one is the culprit?