If properly kept refrigerated and sealed the sauerkraut will usually last 3 to 4 months.
But when sauerkraut goes bad it will start having a foul smell to it or a very sour smell which is very off putting and may make you vomit or want to vomit.
The bad sauerkraut will start changing colors and getting more brown in color when it goes bad.
The texture of the sauerkraut will also change when it goes bad and if in doubt just throw the sauerkraut out to be safe.
But if you take a bite of the sauerkraut and it tastes bad then that's one sure way of knowing the sauerkraut is no longer good to eat.