Most of the existing replies seem to be ignoring/ignorant of the fact that
the OP is talking about a STOVE with #6 wires. There's no way to plug in a
light, radio, etc. etc or even use a Radio Shack circuit tracer without more
work and risk than the OP should take, if he's asking this question.

Simple answer: As someone else pointed out, it will be a large, double-pole
breaker. You can immediately rule out all the single-pole breakers. Then,
follow the following steps:

1) Turn off Main
2) Separate the wires, making sure you have two exposed bare ends (but not
touching!)
3) Turn on main, and with a neon tester (cheap $3 gizmo available anywhere)
confirm there is power on the wires. This will also ensure you're using the
tester properly (If no power now -- STOP, you're either doing something
wrong, or the wire is disconnected from the panel.
4) Now, turn off all double-pole breakers. Confirm there is NOT power at
the wires. (If there is STOP).
5) Now, turn on each double-pole breaker one by one until the tester lights
up. That's your breaker.

Works best with 2 people within hollering distance (or cell phones/etc) but
can be done with one person if you don't mind many trips back and forth to
the panel.

This is the cheapest safe method to do the job. It's very safe if you
follow the above exactly and never touch the wires unless you've confirmed
they are dead with the tester (even then, avoid touching them).

-Tim