On your custom PCB, without Teensy plugged in and without any power (if it gets power from any other source) try using a multimeter to measure ohms between 3.3V and GND.
Remember, ohms measurement isn't magic. Your meter sends a test current through the wires and measures the resulting voltage and then uses ohms law to compute the resistance. It's meant for measuring only an isolated resistor. When used on a PCB, keep in mind that test current can and (probably) will flow through various circuitry. Capacitors measure as a low resistance that gradually increases as the test current charges the capacitor. If the current flows through chips, usually you'll see a fairly high impedance on the meter, but it's not actually resistance since the current is flowing through various P-N junctions inside the chip which (usually) gives a diode forward voltage. What sort of number you actually see varies with different multimeters depending on their test current and which range they're configured to use. But if 3.3V and GND are shorted together on your PCB (my guess....) you'll see a very low resistance on the meter.
You could also use DC voltage mode to measure the 3.3V power while Teensy is plugged into the PCB. If it's close to zero, that's a sure sign of a short on the PCB. If you get something like 0.6 to 0.9 volts, that's a strong sign something on the PCB using 3.3V power is wrong, like a 74-series logic chip installed backwards.