@monorail See this picture: https://i.stack.imgur.com/eHVnK.png
It's not 100% reliable but on most LEDs, the anvil is the negative side, and the post is positive.
You can always check the datasheet, too, but I find people don't often remember the exact part number, or even brand, of the LEDs to look it up.
@Rosemary yeah, these ones came in an arduino kit so i have no idea about part numbers or anything
@Rosemary i don't know what you mean by the "anvil", i just know that i was taught that there will be a flat edge on one side of the plastic bulb part