Using a different background image won't help notice that you've accidentally booted into a clone, because a clone will have the same background image. My main drive has a blue background on the desktop and the clone has a gray one. I can, therefore, tell immediately which one I'm booted from.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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