What can pilots do to spot the onset of hypoxia? If there is a loss of/lack of air pressure at high altitude, without an explosive decompression (for instance as on Helios 522 or N47BA) are there any signs that pilots can look out for that they might not be getting enough oxygen?

Does an audible alarm exist on any commercial jets when cabin pressure is at a dangerous level?

It's my understanding that the brain is impaired, and reaction times are increased with oxygen deprivation. Are there any techniques to combat this, after noticing hypoxia onset? e.g. slowed breathing
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.