Erm there should be some sort of writing on the cable, is there anything?
There's definitely nothing written on either of the long-enough cables I have, the too-short cable they've sent me does have 5e written on it. A suitable long-enough cable would cost me about £5, guess I should just take the chance and buy it.
Yes, it’s written on it.
But it’s more a measure of performance than signal strength; quality comes into it as well.
I check Ethernet cables by routing them through my Gigabit switch, which lights up differently if it’s seeing a gig, or just 100 Mbit. Because even Cat 6 cables, if badly treated, can become compromised and drop to the lower level of performance.