Spell Penetration Items, do they work? / + a little question about a sylvan sorc

[lotnik_xaos]

I've made a "dragon warrior" (1 scale fist -> 2 warrior -> 2 sorc -> 10 dragon disciple -> rest sorc), and sure, I don't expect high spell pen with this char, that's not the point, but from all the items I've got that should increase it, only Ruin staff works (gives me +4 because I'm chaotic), but I also have this Robe of The True Master, it should give me +5 (I think) and some ring with +2, and they both do not work, I don't mean stacking (would be nice, tho), they don't work even separately, only that staff works.
Let me clear this up, by "works", I mean that it shows some difference in that little table in my spellbook. Are those other items bugged or something?

EDIT:
About stacking spell penetration and DC's, I'm planning my next playtrough with an Elven 1SF/1Rouge (or 2, for Fast Stealth, I love creeping and scouting through dungeons faster than anybody)/Sylvan Sorc + some AT (I think 4 lvls of AT put somewhere) rest lvls with a bow and a staff, what would you suggest for best results?
Regarding spells, I plan for CC for every save, some nice damaging evo (rays/ranged touch for sneak attacks) and necro, 'cause necro feels mighty powerful sometimes <3

[Cheb]

Unfortunately dude (sorry for the late reply), the only way to check is to cast a hostile spell against something with spell resistance and checking the combat log. If you have the spell resistance spell, or an item that grants SR, have fun spell sniping your allies to check it out :)