hello coffe_s
you need a meter and some knowledge. Take the screen off again, and disconnect the display as well. Don't forget the battery - this is extremely important. Use the DIODE mode and check the individual digitizer leads. You can enter the result here (if you cannot interpret, I will help).
another option is finding a good repair shop. It will save you some worry and having to (perhaps) buy tools. However, if you prefer to do it yourself (which I urge) it is doable up to a point and quite simple. However, if you do reach the wall, I strongly suggest a repair shop. The iPad is more delicate than people think.
* take care,