Installing rads - need to know feeds & returns?

[OhPinchy]

I ran in pipes for 3 rads in an extension a while back and am only now fitting the rads due to a big delay from the supplier of the vertical rads.
I branched off from the existing heating system and ran 2 pipes to the furthest rad, with branches along the way for the 2 other rads. I have the pipes centred correctly and coming out from the wall at the right spots but am not sure which ones are the feed and which are the returns?. How important is this and what is the best way for figuring out which is which?

 

[davidoco]

Some (not all) rad valves allow flow of the water in both directions on the valve so you can safely fit the valve and then worry about which side the hot/flow is coming from afterwards. see this one

I see you are fitting a vertical rad which may have "designer" like fittings. If the worst comes to the worst you will have to drain down again and swap them.