there are many tools that will allow you to check phase. You can use meters like the waves PAZ analyzer, spectrafoo, and you can also use the mono test. Synths can have phasey sounds. I'm sure one of the vinyl cutters here will elaborate more.

What you have to avoid, if possible, is having too much "stereo" or difference between the left / right channels. This is especially a concern in the low frequency area up to about 300Hz or so. If there is too much out of phase information the cutterhead will attempt to cut a very deep groove, possibly ruining the side and/or the cutting stylus.

Often cutting engineers have to control that aspect of the sound with a "crossover" which makes the signal mono below a preset frequency. This aids in making the cut a more uniform depth.

None of this is an issue for CD replication or mastering, just vinyl. It is one of the major things I pay attention to because lots of the young engineers & producers may not be aware of that vinyl format limitation.

In the mix (for vinyl) it would be helpful if the bass frequencies were put dead center and not panned to either side.

Many thanks for that.
I was aware of the problems with the stereo of the bass. (£300 to find out we had messed up the mix! lol) But on the other recording it was a mid tone lead synth sound that caused the problem. At the time I was unexperienced and too embarassed to ask the engineer what he meant as I didnt really understand the process.
So can you still get these problems with a higher pitched tone if there is wild stereo action in the sound(as may well have been the case)?

I suppose that's possible. But midrange usually doesn't cause me problems. Maybe there were other factors?

What kind of music is this? Are there any visious high frequency sounds?

The sounds in question were solo synth sounds which were being cut on vinyl so that they could be skratched on turntables into another composition, the sounds were pretty far out with a fair degree of modulated pitch (its going back a little so I'm a bit foggy about what I did exactly).

The sound source was a Roland JV1080, going into a Spirit 328 digital mixer. There may have been some compression and reverb added from the desks internal fx.

Phase used to be something that was checked at each stage of the recording process ... but this practice has faded as CDs and mp3 have become the format of choice.

You can check the phase of your synths and other tracks by colapsing to mono when tracking and mixing. On stereo tracks with extreem out of phase material, you can remedy the problem by panning the left right into about 9 and 3 o'clock positions ..