Latest (rpi-update) firmware includes a new gencmd:
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$ vcgencmd read_ring_osc read_ring_osc(2)=2.947MHz (@1.20V)
I believe that it varies from around 2.5MHz for slow silicon up to around 3.25MHz for fast silicon.
Having a higher number implies a higher level of overclock is possible.
So, if you are unable to achieve a high overclock and the ring oscillator shows a low value, then you are probably limited by the silicon.
So, if you are unable to achieve a high overclock and the ring oscillator shows a high value, then the problem is probably down to the power supply.
In theory it could be possible to create an sdcard image that picks an optimal overclock based on this reading.
Note, the number increases with core_voltage, so it's best to measure this with overclock/overvolt disabled, so everyone is on the same scale.
I'd be intested if overclockers can post their ring oscillator value (at 1.2V), and the highest stable overclock settings, and we'll see what the pattern is.
On by B+ I get:
seems stable, so this Pi seems pretty fast.