Home DIY

Yeah, like musa said, water down some emulsion and paint the wall - it'll show up where there are surface imperfections much better than just having bare plaster, then you can go and patch them all up with some filler - for the tiny blemishes up to about 1mm deep, fine surface filler is the stuff to look for - toupret do a good one in a tub with a blue lid. For deeper things like holes left by rawl plugs etc just use regular filler - it might sag a bit when it dries out, but you can always fill the remainder with the fine stuff.
Even if you're putting lining or wallpaper up, you'll be surprised what shows through. You want it as flat as you can get it.
Once you're done, run your hand over the wall to find any little poky-out bits and nip them off with a scraper or a sanding block.