How to Test a Breakout Board for Your Raspberry Pi
To test your homemade breakout board for the Raspberry Pi, you need a multimeter, a device for measuring current, voltage, and resistance. You can obtain a cheap one from a thrift shop for less than $10.
First, select resistance or continuity on the switch on the front of the meter and make sure that there are no shorts between adjacent wires in the ribbon cable. You will know it is a short when the continuity indicator bleeps or the resistance measurement shows zero or a very low ohm reading (less than 10 ohms).
Next, plug the cable into your Raspberry Pi with the ribbon running away from the board and the polarizing bump pointing into the board. Do not power up your Raspberry Pi yet.
Check that you have continuity between the five ground connections. Now power up your Pi and set your meter to measure voltage. Put the black or negative meter wire on a ground connector and the red or positive meter wire on a 5V block. You should see close to 5V.
Next measure the voltage between the ground and the two 3V3 connections in turn. Again, you should see a value very close to 3V3. If these checks fail, shut down the Raspberry Pi disconnect the cable and check all your wiring.