Over the vast years of collecting bricks of various sizes and sets, I find that during my free builds I run low on a specific type of brick. How can I go about identifying what the brick is named and what resources are available for this identification process?
Check for visible part numbers
This usually only works if you have the physical part in front of you.
Look for a 4 or 5 digit part ID on the element. These are usually printed on the inside. A magnifying glass and good lighting can be helpful. Many bricks will also have other numbers printed on them, but only the 4 or 5 digit numbers are useful for identifying parts.
If you are able to find a part number, search for it on Rebrickable or Bricklink. Rebrickable tends to work well, as it allows you to search by the official Lego part ID printed on the brick, and this number can be different from the Bricklink ID for historical reasons.
Check a set inventory
If you know which set your part comes from, or you know of a set that contains the part, you can look at the inventory of that set on Rebrickable or Bricklink. Simply visually scan the inventory for the part you need to identify.
Search a part catalog
You can search for parts on a number of sites. For example, Rebrickable provides searching by part name, ID, category, and a color that the part has been produced in. This can help produce a relatively small number of parts to look through.
Taking some time to familiarize yourself with the part categories on whichever site you choose to use is also very useful so that you can quickly limit your search to a few hundred parts in one category.
If you don't know... just take a pic of it and ask here, we'll find it :-)