Hi John,
The honest, correct answer is "try it and see." Seriously! W/O an AGP/PCI lock, raising your FSB also raises those bus speeds from their specified 66 and 33MHz, respectively.
"How far" you can go depends on component sensitivity. For example, your soundcard MIGHT NOT care if it's bus is at 38MHz, but your NIC may crash and die at 38 or even 35MHz.
Hard drives as well are affected by raising bus speed. Data loss or HD DAMAGE is possible. Maybe not probable, but the risk is there.
This is why everyone loves the NForce2 chipset (at least for AMD CPUs!). That built-in lock is simply God's gift to Geeks.
The "signs" might be there and they might not be, unfortunately. Something as simple as the soundcard not working/being detect is a good one. The system not booting is a very obvious one.
Hope this helps.