What can pilots do to spot the onset of hypoxia?
Yes, all pressurized aircraft have cabin altitude (= altitude at which the same pressure as currently in the cabin normally occurs) indicator with alert when it exceeds safe value.

On Helios 522 that alarm did sound. The crew however mistook it for configuration alarm which only makes sense during take-off roll and thus considered it spurious. When the technician they called realized it was cabin altitude warning, the pilots were already too confused to carry out his orders.

The symptoms are already listed in the other answer. The problem is that the initial symptoms feel just like fatigue. One starts to feel sleepy and at that moment one might not realize it could be due to hypoxia as mental capacity is already reduced.