I bought a cheap knock sensor off ebay and use that as an input for a headphone amplifier.
These you can buy for a small amount or make yourself
To buy an example:
http://www.ebay.nl/itm/SMSL-sAPIII-T...item1c4557923b
To make yourself an example:
http://tangentsoft.net/audio/cmoy/
Or something like this:
http://www.ebay.nl/itm/HA-PRO-Single...item3a8d9f2fe8
While tuning you put on a headphone that amplifies the sound of the knocksensor. Use a exponential gain for your amplifier as that makes up for our hearing. You will hear liniair increase. The knock kits are just the same but more expensive with much more features. The question is what do you want to use of them and is this enough for yourself. For me it is!