Connect the LED along with the 270? resistor and wire them to the 3.3V rail according to the circuit diagram. From the datasheet, the shorter pin is labelled 'anode' and should be connected to the positive rail. The datasheets are usually found on the product page for electronic components.
I've seen them all ways, whether comparing lead-lengths, or vs the flat, or looking at the internals ("small part" vs "big part").Just rig up a low current (5V, 2K) test and find out yourself.Assume nothing.
The real crime is how many tutorials rely on a convention, not a standard.I teach electronics classes at my local TechShop. Before each class, I purposely cut the anodes to be shorter than the cathodes, then while teaching how to use a multimeter, I ask the students to identify the anode and cathode... Very few realize they should use the multimeter, not their eyes.
Boffin bought a load of "reverse polarity" LEDs a while back, got a good price from the factory as I recall. Pretty sure it was a manufacturing error.
I've always looked at the led itself look carefully and you see both the anode and cathode and the bonding wires, the little segment bonding wire larger segment, all of the 5mm / 3mm leds i've ever had has worked using this method the small block is positive, negative from the larger segment.
The reliable indication is the flat ornotch on the plastic body which is always the cathode side.