Current 79 Nishiki Royal, Jeunet 620, 59 Crown Royal, notable previous bikes P.K. Ripper loop tail, Kawahara Laser Lite, Paramount Track full chrome, Raliegh Internatioanl, Motobecan Super Mirage.
There really isn't any way to clean the insides of cable housing either they are fairly good or better, not so good and need to be replaced. More than likely yours are just fine stop worrying and just stick the cable through them.
1992 Marin Eldridge Grade, 2007 Kona PHD and 199? Trek 1000 (current build project)
Before now, I've just used the thin red plastic straw that comes on a can of WD40 to blast the liner out from one end to clean & degrease it, and then a bit later used a can of thin spray grease the same way to lube it - you can see it firing out of the other end.