How to determine what 3-way switch system was used?
Take a look at the diagrams in the Wikipedia article you linked to. These will tell you what you're dealing with. There are some big red flags that are easy to look for:


First, kill the power to this circuit at the panel. This will allow you to poke around in the wiring safely. Use a non-contact voltage tester to make absolutely sure there's nothing live in the box before you start work.
Now, examine the switch that is getting the "hot" lead from the panel. This may be hard to identify, especially with K&amp;T wiring; I would recommend labeling where each wire to the first switch is connected, and then disconnecting and separating all wires from the switch. Then, turn the power back on and use a non-contact voltage tester to identify the "hot" wire. It will also be helpful to identify the "neutral"; it will be the wire between the panel and this box that completes the circuit but does NOT carry voltage when disconnected. 
With the power to the circuit back off, and the hot and neutral identified, look at where each wire connects. If the neutral wire coming from the panel is connected to any terminal on this switch, you have an illegal circuit, whether it's Carter (the hot would also be connected to the switch) or somebody just swapped hot and neutral while messing with it (the hot would then be "hard-wired" elsewhere in the box). This is unsafe, because of the "switched neutral"; if the switches are positioned such that the neutral is disconnected to break the circuit but the hot still has continuity, even though the light is off, the socket is still energized from the "hot" side and can give you a nasty or fatal shock of you touch it.
Pretty much any other system that could have been used is "safe", as long as the neutral is never the side being disconnected (or the design of the switch guarantees that BOTH sides are disconnected simultaneously, such as DPDT switches; these are common in electronics but rare in home wiring). 
If the hot was connected to a terminal labelled on the switch as "Common", or somehow identifiable as such, and the neutral is not connected to the switch at all, you likely have Traveller-system wiring, which is perfectly safe and has been the best practice since "single-conduit" wire like the current Romex wire became available. 
If the hot side was connected to a terminal other than the common terminal, there is another wire coming from that same terminal going to the second switch, and the neutral is not connected to the switch in any way, that's California-system. As long as both switches are wired to properly follow the same system (understand this is an older system rarely used anymore, and may confuse even a competent electrical pro), it's fine to keep using.


Understand that this is all rather moot according to most modern building codes. It is illegal, under any circumstances, to:


Install new knob &amp; tube wiring systems; K&amp;T hasn't been code-compliant since the '40s.
Re-route existing knob &amp; tube; even if you're using pre-existing wires, moving them to a new place counts as "new work", usually requiring a permit and inspections which will never pass if they involve K&amp;T.
Add new wire runs to a knob &amp; tube system; You can't add K&amp;T because of the prior points, and you can't join the current Romex wiring to K&amp;T because it's a fire hazard in itself and because it's misleading to anyone who's looking at the endpoint; they'll assume the whole circuit's Romex, join to it, overload the circuit and burn the house down.
Run more than 10A through K&amp;T. K&amp;T is quite safe from a fire hazard perspective if you keep it under 10A. However, you can't run much on 10A; you'll blow a fuse with the average modern vacuum cleaner. Fuses are interchangeable enough that it seems like a no-brainer to simply put in a larger fuse like 20A; this is how K&amp;T wiring causes house fires (and the reason neither K&amp;T nor fuses have been acceptable by code since better stuff became available).
Use copper-rated electrical nodes (switches and outlets) with aluminum wire of any type including K&amp;T; quite a bit of K&amp;T is aluminum, which has higher electrical resistance and thus gets hotter for the same amperage than copper or copper-clad wire, especially at junction points like screw terminals. Cu-only switches/outlets are not rated to withstand this heat; you have to pay extra for Al-rated stuff.


Given all the above, if you're doing anything more than an in-place replacement of a light switch, you could be in for an expensive renovation. If you are moving any part of the K&amp;T circuit, it loses the grandfathered status, meaning the whole circuit has to be taken out back to the panel and replaced with Romex. That may also require the panel be brought up to code (I can't imagine you having K&amp;T and a modern breaker box); if you need to do that, you'll likely invalidate ALL the K&amp;T in the house and have to rewire the whole thing.

For peace of mind, if it were my house, I would contact either my home insurer, a trusted local licensed electrician, or the county clerk, and get the straight dope on where the upgrades would have to stop in certain situations. If messing with the wiring is going to result in you needing a full house rewire, DON'T TOUCH THE WIRING unless you have the scratch to take it all out, even if it's illegal; it's not your fault it's illegal until you change something.