The "rule of thumb" is, there is no rule of thumb, but there are a couple of relatively safe bets.
1) There are really only two main issues with operating speed: bearings and cooling. Many motor mfrs don't really bother stocking and using different bearings for the different speeds (6 pole, 4 pole, 2 pole) so most of the time you can run a motor at the 2 pole speed, i.e. 3600 / 3500 RPM nominal. Some cooling fan designs are fine with the higher speed, some will not move more air however, and if the motor is being over loaded, it may become a problem (see below).
2) Motors are not really made for 50 or 60Hz, but they ARE made for a specific V/Hz ratio. As long as you maintain that same ratio, the torque remains the same. But if you change it, you change the torque profile and can get in trouble, especially if the motor is marginally sized. So for example in your case, your motor was rated for 50Hz operation, but you didn't say at what voltage. Let's assume for now that it is 230V 50Hz as designed. That gives it a V/Hz ratio of 4.6:1. But you have been feeding it 60Hz, which lowers the V/Hz ratio to 3.833:1, so 17% lower, the equivalent of lowering the voltage by that much. Torque relates to the square of the applied voltage, so you have lowered the torque to roughly 69% of what the motor was originally designed for. At the same time, you have increased the speed by 20%, so now you have 69% torque at 120% speed and because HP is a function of speed and torque, you have about 84% of the original HP available from that motor. If the motor was over sized to begin with, might be no problem, as it appears to be your case. But it is by no means a given.