What are the signs that a head gasket is going?

Engine knocking followed with coolant loss

 

Have you cleaned the EGR and intake manifold? It's a sand pounder, to just sit around waiting for the "inevitable", when some preventative maintenance would likely prevent the head gasket failure. 100K miles is not too soon.

More head gasket failure symptoms for 3rd gen Prius:

1. Check Engine Light with code for misfire, in particular on cylinders 1, 2, or both. (p0300 is general/random, then p0301, P0302 and so on for individual cylinders)
2. Evidence of carbon monoxide in the coolant. (there's a test kit for that)

If you google, there's something like 6~8 ways a head gasket can fail. There's various passages in the head gasket: the combustion chambers, oil passages, coolant passages. The head gasket can fail in ways that allow any of those 3 to leak into another, and/or to the outside.

 

Typically....a loss of coolant. Monitor the coolant reservoir level.
Misfire...uneven idle.
Engine oil....a milky color on the dip stick.

Next level, perhaps an engine error code will be displayed, borescope piston top - clean piston head is a sign, sparkplug reading may or may not be a lead, compression and/or leak down test can also be an test for head gasket leak - but can also include valve sealing issues.

Not sure your engine oil burning is a head gasket failure sign, IMO typically it is a coolant loss.

Good luck.