If you don't have a locking, LSD or spool axle, then you have the stock open differential. You can go to sites like "how stuff works" to see cartoons showing how an open differential works. but the short story is, if one wheel starts slipping, that wheel will get all the power. Whereas with a locking differential, both wheels get power regardless whether one is slipping. In real muddy situations, an open diff truck will get stuck before one with lockers.
If you live in the US, it's most likely a US model. The toyota locking differentials that came on some non-US truck would be cable actuated. There would be lever(s) behind the emergency brake to engage the locker. If a previous owner installed an aftermarket auto locking differential the only way to tell is to look at the locker it's self or do the test mentioned above.