Here’s my contribution to this stone soup. Since I’ve used these photos in so many other posts, why not make use of them here?
What I can add to this is my favorite mantra "If I had a dollar for every time someone posted… 'I swear I cleaned the plate with {fill_in_solvent_used} and scrubbed it {fill_in_damaging_cleaning_pad_used}" I’d be rich!!! 
€:euro:
If you think your plate is clean, try these tests.
Which one of these do we think is clean? The one on the left, or the one on the right? 
Still not obvious? Well on a textured plate, you can’t easily see fingerprints. However, try this on a smooth plate and it becomes more obvious.
Note: Fingerprints generously applied after eating a cheeseburger. 
Click to zoom in. This was on the Bambu high temp build plate which is unforgiving when showing plate contamination like fingerprint oils.
How to do a quick first layer test.
First, unless you genuinely prefer downloading, there’s no need to download a first layer test, as the process can be done faster than the download itself. Additionally, you can customize the test size and position smaller patches on your build plate to check specific areas for contamination or damage, saving time and filament.
Note: I am using Orca Slicer 2.2 so your menus may look a bit different in Bambu Studio but the same techniques apply to both slicers.
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Right-click and start a cube primitive.
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Scale the primitive to the size you want. 225x230mm covers most of the plate but 150x150 is faster and 100x100 faster yet, you decide. The key is to make sure the layer height is 0.28-0.5 so that the slicer only prints one layer.
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Slice and print.
Differences in printing times.
Think your plate is clean? Print the entire area and see what your filament tells you. There is no hiding a dirty plate when you perform the first layer test.
Neat trick to change up the pattern.
Sometimes, a plate may be damaged for reasons unrelated to plate hygiene, or there could be a bed-leveling issue. An alternative test is to use a circular pattern, which creates a different type of first layer that can reveal distortions in layer adhesion, plate leveling, damage, or contamination. Plus, it looks cool!!! 
