There's no good way to do this. You could (in theory) get a spare electrical box, wire it into your main box such that it had access to more current than the breaker under test, and then install the breaker into that spare box, cause a short and watch it trip.
Alternately, you could install the breaker, overload the circuit in question (borrow all the neighbor's vacuum cleaners?) and watch it trip (and hope nothing goes wrong, causing a fire and burning down your house).
The first is annoying, the second is dangerous (because you have no idea if some previous owner used sub-standard wiring somewhere in your house). There is no good option for the homeowner to test these things.
While I don't think we can make too many general statements, we CAN say that the products distributed in the US are subject to various electrical codes and that the manufacturers design so that if they fail, they fail open. Realistically, you've probably got a lot more to worry about from your appliances, than your breakers.
Just buy your breakers from a reputable retailer, check for damage before installation, and you'll almost certainly be fine.
A double-pole breaker with a "60" on each half will trip at 60 amps. The reason it's two connected breakers is to give the circuit 240V potential, by using two opposing AC phases. These do not "add up" to more current -- you're using them to double the voltage.
So if the heater requires 240V power at 120amps, you will need double-pole breakers that can provide a total of 120amps, e.g. 4 30A double-pole breakers as Tester101 suggested.
An electrician will be able to tell you if your service panel can accommodate this extra load -- it's a lot, but your 200A service might be adequate, depending on the normal load elsewhere in your home.
Best Answer
As ChrisF mentions, any mechanical device causes some wear when it operates. With that said, let us start by checking with the National Electrical Code (NEC).
So according to the NEC, circuit breakers can be used as switches with some provisions.
If the circuit breaker clearly indicates the "on" and "off" position ("on" being up in vertically oriented situations), and the breaker is labeled "SWD" and/or "HID". Then the circuit breaker can be used as a switch, as far as NEC is concerned.
As for whether or not using a circuit breaker in this way causes damage to the circuit breaker, we'll look to the NEC's definition of a circuit breaker.
Notice it specifically mentions "without damage to itself". So in the eyes of the NEC. A device used manually or automatically to open a circuit that causes damage to itself, can not be used as a circuit breaker.
More evidence is provided in this pdf document from Schneider Electric describing the markings on a circuit breaker.
While the information presented here, is based on information that may only pertain to the United States. Other countries will have similar rule and markings, to indicate which devices can be used safely in different situations. Circuit breakers rated for this use are tested for durability similarly to switches, and can safely be used to manually open and close a circuit without too much wear and tear.