Crystal "calibration" - how to (a simple way)?

[Archangel]

This is one way of doing, thank you Joe S. But this can take a lot of time...

Isn't there another more "scientific" way to measure the correct oscillation frequency?

I found some information in previous posts (can't find it right now) about measuring the crystal but there was an issue with the capacitance of the probe making the result inaccurate.

If I make a short code toggeling a port and me measuring it with my (poor) oscillo, would the result be somehow reliable?

Sure there is, hook up a frequency counter, an accurate one, time your circuit's operation and log both entries, adjust as in above and time it again and compare OSC speed to circuit change, you then have a ratio to work from. OR use an RTC.
Oh yeah, I thought of one more way, if you have an accurate radio reciever, you can check for the oscillators harmonic frequencies at different dial settings, 5,10,40,80 mhz and so on up and down the dial. Or use scope to form Lissajous patterns. example link http://www.egr.msu.edu/classes/ece48.../lissajous.htm

Just curious Roger, why all the fuss for a battery charger?
HTH
JS