Find a friend with an oscilloscope, and some circuits you can probe (these can be anything you have available).

Probe the circuits at various points with your new oscilloscope and the borrowed one, and check they see substantially the same thing. In most consumer devices, you will find a range of analogue and high frequency signals, just stay away from the mains power supply until you are competent to work in that area!

This might not work for you, but is probably the simplest and most reliable method outside of having a full calibration laboratory, solutions involving 'build a simple circuit' will always leave you wondering whether the circuit or the new oscilloscope is doing something unexpected.

Find a friend with an oscilloscope, and some circuits you can probe (these can be anything you have available).

Probe the circuits at various points with your new oscilloscope and the borrowed one, and check they see substantially the same thing. In most consumer devices, you will find a range of analogue and high frequency signals, just stay away from the mains power supply until you are competent to work in that area!

This does depend on temporary access to another oscilloscope, but is probably the simplest and most reliable method outside of having a full calibration laboratory. As a bonus, it will let you test your oscilloscope capabilities and give you confidence that you can use it correctly.

Any solutions which involve 'building a simple circuit' will always leave you wondering whether the circuit or the new oscilloscope is doing something unexpected.