Really stupid question about envelopes

[WhoLurvesOrangeSoda]

I know I should probably know this at my age but how do I know if an envelope is prepaid?
I need to send something to my solicitor, the have provided an envelope with their address on... how do I make sure it doesn't need a stamp?

The address will say free post or there will be a 1 or 2 in the right hand corner where a stamp would go.

Thank you!
It doesn't say free post but their is a number 1 with two black lines where the stamp should go... that means it's paid for?
(Just want to be doubly sure before I chuck it in the post box)

*there!
Gah... can't function this morning, lack of sleep 😴

It is prepaid.

Wonderful, thanks both for your help!

Even if it’s not pre paid, the company will have to pay on receipt

The Inland revenue does not pay if you forget to put a stamp on it .