Need some equipment.

I take a DC voltage source and then adjust a square wave generator to that voltage and go up seeing if the amplitude stays with the changing of the V/div. control. And move the position control to see if the waveform is linear at all portions of the screen.

Keep going up in frequency, I happen to have a generator that stays stable at any frequency, it is a Wavetek and operates on a different principle than most. Most do keep a square constant, but not necessarily in sine wave.

Next you need a freq counter, set the generator accurately to a 1,000Hz square, put the scope on 1mS/div. Adjusting the horizontal position to where the first cycle lines up with the graticule, check the rest. They should also line up and the vertical parts of the square wave should be on graticule lines all the way across. If they run off, the time base or H size is off. To determine which, switch to different frequencies ad time bases, always with the freq counter on it. If they vary it is the time base, if they constantly run off one way or the other it is the H sweep size - or more professionally put - deflection.

How accurate you need it depends on price. I got one I don't pay much attention to, the time base or deflection is slightly off but I am not measuring frequency with it. The amplitude reads right and that is enough because I use it for everything. Even DC, it is much quicker because usually I only need to know a source is there.

If you need a scope that is totally accurate all the way you need an old Tektronix.