The steplist has a search box (B) that filters the parts shown in the steplist. 
Simple searches
You can enter a BrickLink ID or salient search terms. For instance, to find all the Plate, Round 2x2 in the current model, you can enter:
- 4032, its BrickLink ID,
- Plate round, or round plate, its main traits,
- or 2x2, its size.
You can also search by color names. For instance, blue will show only the parts in one of the colors with blue in their name.
The parts will be filtered at each keypress, so you may not even have to type everything before finding the parts you want.
Special searches
Show parts in unavailable color
This feature is available through a shortcut (by default undefined) or with the special text =warning().
The parts listed are the ones with a yellow exclamation point, those that are not found on BrickLink.
Show parts not available in the selected palette
This feature is available through a shortcut<link> (by default undefined) or with the special text =!CurrentBuildingPalette().
It looks at the current parts palette (that you can choose in the Building Palette) and only shows parts that are not available in the palette.
It can be useful to search for parts that are not available in an official set or in another of your custom palettes.
This search doesn’t care about counts. If the current palette is a counted palette, as an official LEGO set palette, this feature will not show if you used more parts than were available. It will only show the parts (color and type) that are not in the current palette.