I think we may be missing the point here. I think the OP is concerned about the reliability of the oscillator, not its precise frequency. The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all. I think you would select the capacitors by starting with the datasheets and a little math, then doing final testing with various values and testing both the crystal voltage to make sure it isn't too high and then the startup characteristics over the range of temperatures you expect the device to be subjected to. If you want to measure and view those things, you need a probe such as the one I mentioned--and even that is going to have at least a small effect.
This isn't tuning a transmitter. How accurate do you thing the average internal MCU oscillator circuit is anyhow, even with a proper crystal?
Yeah I think a number of replies are going way too complex for what I am trying to do.
I am not suggesting I am the best at electronics or testing anything, and the fact I missed the 'glaringly obvious' fact that xtals cant work at 400hz or 1.2khz when they are meant to be 12Mhz, I apologise. You are superior than me.
Turns out I was looking too far out on the scope and had to 'zoom in' a bit, and the 12Mhz then came up fine. I did find it odd that it wasn't locking on to the 400hz very well, despite my trigger, and I guess it explains why.
I don't do this type of work daily.
I have a goal I am trying to reach and I am doing the best I can to try and achieve that goal.
We have systems which are all running fine. The 12Mhz isn't in question really, the products work. I am just trying to determine if the brand/model of crystals we have currently are running the best they can based on the loading capacitors in the circuit. That is it.
I know crystals work at quite a lot of values within reason, but then become problems when the temperature goes up or down, and other factors like that. Nearby noise etc.
We have had some problems in the past when our product is used in particular environments where there are high noise devices, and our products can go a bit weird. I am just trying to work out if a factor like our loading caps on the particular brand of xtal we are running at the moment, is contributing to these edge cases.
IF working it out from the xtal datasheet, is it a case of "Capacitor Value = 2x CL - 2x Stray" ? or something close to that sort of formula?
This is a 12Mhz xtal running a microprocessor. Not RF/Radio/Wizardry
Thanks everyone for trying to help, or point out how useless I am.