Symmetrical Clipping

Yeah that´s what a CRO is. You monitor the output and with a test tone from either a generator or a test cd you try to bias the amp so when clipping it clips even (the same ) on top and bottom of the wave form. It not a cheap set up But it´s the only that does the job. It would be easier if you could use someone elses. Like from a Lab or a friend .
Yes, CRO is oscilloscope.

If you got one, then you should hook up a signal generator to the input of your amp. (or if you have a test CD with tone signals). Hook up your CRO to the output of the amp. Slowly turn up the signal, you should start to see the waveform on the CRO grow larger and larger but keeping a good sine waveform.. At some point, you should start to see the top of the sine waveform flatten out, the amp is clipping. At such stage, turn your bias adjustment VR until you see that the flatten portion of your sine wave is symmetrical on the top and bottom. You are done.
Not to be pedantic, but...

How I would do this more accurately is stretch the time/div out as far as you can and still see the top of the wave. Then measure the clip's length. Then switch your trigger from + to - and measure the bottom.

That should eliminate some of the guessing.

My experience is that stretching too much on time scale will flatten the sine waveform so that it is not so easy to tell where it start to clip. Keeping 2 full cycle on the CRO screen to me give the best picture.

This tuning is not intended to be mV type of work, so extreme accuracy is not the prime concern.
A few years ago when I first started I just bought a cheap oscilloscope off Amazon for about £30. Downloaded a 1khz sine wave to my phone and used the phone as the signal generator. I think someone else has said that a triangle wave is probably more accurate for symmetrical clipping. Do you have pots for adjustment. What amp is it?