Stick a piece of ground glass to your film rail, ground surface facing the lens. Keep the shutter open with B and a cable release. Check for infinity focus looking at the image on the ground glass using a 10X loupe while focusing on the moon or anything that has sharp and contrasty edges more than 400m away (what I do when I can't find the moon). Check to see if the RF agrees with the ground glass. If not, adjust the RF till it does.
Similar things can be done for targets at 1m, but that would be slightly more complicated.
I haven't cracked open my R3A yet, but I am sure many of the members here have had a go at it. I remember seeing a post about this.