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?