How can I determine voltage and current limit of protoboard?
Since the resistance (R) of any plated through-hole or copper trace on a proto-board is very small, no matter the voltage, not much power (P=VxI=I^2xR) will be dissipated.

What you are really asking is the power rating of your proto-board. Power is related to heat which will be the cause for damage. Most proto-boards will be fine for many applications, even high voltage. If you are working with high voltage, your concern should be for your own safety rather than the proto-board.

In conclusion, proto-board should be able to handle high currents because the resistance of a copper trace or plated through-hole is very low.

P=Voltage x Current=Current^2 x Resistance -- if resistance is small in this equation, then you have low power through the copper trace / plated through-hole



Your concern should now be focused on the power rating of the rest of the components in your circuit.

In a PCB, power ratings of a trace will depend primarily on the width and height of a trace (height in the US is usually defined in oz, ounces of copper per unit area).