Ok, so here is how you can know there is absolute truth- the law of non-contradiction.
Basically the statement "there is no absolute truth" would be true if there was no absolute truth, but since it would be true, then there is an absolute truth, and therefore the statement would be false.
The statement "there is an absolute truth" does not contradict itself. If its true, then it remains true. Example:
I say all philosophies on life are equally valid. Therefore your philosophy on life is as valid as mine, and as true as mine. However, your philosophy on life is "not all philosophies on life are valid, and definitely not equal." They complete contradict each other, and simply cannot co-exist.