How can I find the traces I need ?

Tachometer hacks to calculators are pretty cool examples of field hacks! Wireless antenna, Q and field pattern acquisition are good ones, too. Strictly speaking you could pop the membrane, feed as strong a signal as you feel safe in (say, 2V trapezoid wave up to 80ma) and use a polarimeter and camera to see which trace is affected most, but if what you have handy is emery paper and a DVM, and nothing from TI as schematic or forum time, strip enough of all traces you want to consider and get it to beep when the sort of key_down deflection you hope for comes (while whaling on the key; or find the polling and deflections topside first.)