Forgot to mention that you do have a sort of tensionometer. Tap the spokes with a plastic screwdriver handle and listen to the tone. Of course you need to know what tone to listen for. Can't help much there. There's some online freeware piano game programs and if you download one and get it going then I'd say you're looking for something around the G or A below middle C on the DS and maybe the E to F below middle C for the NDS spokes. At least tapping on the keys of Electric Piano 2.5 these sound pretty close. Get it from...

http://www.pianoeletronico.com.br/index-en.html