All lorries should be required to show their weight (without goods) via a symbol (sticker) attached clearly to rear and front of vehicle. Until this is done here are some pictures to give people an idea of what is/isn't 7 tonnes or greater.
Of course this doesn't take into account loaded vehicles.
The pictures below provide an indication of what each weight of lorry looks like.
4 Tonne Lorry
7.5 Tonne Lorry
As for skip trucks, the nice lady at RTS Waste Management tells me that a skip truck loaded with an empty 12yd skip weighs betw 9.6 - 9.8 tonnes. So even with a standard 8yd skip the truck is over the 7 tonne limit for ladder roads