What are some indicators that the guavas are ready to be harvested?

Guava trees are usually ready for harvest within 3-8 years after planting. Once the tree is fully productive, it can get up to 80 pounds of fruit a year - and in some areas, even get two harvests a year. Guava is usually ready to pick once it changes color. Red guava will turn from green to yellow. White guava turns from deep green to light green. When smelled, the fruits should be sweet and musky. Finally, squeeze them. Are they firm, or do they have a slight give? If it’s the latter, they’re ready to pick. Timing is the key because if the fruits are picked early, it won’t taste good, and if delayed, they become a target for fruit flies. 

Source:

Growing Guava: The Complete Guide to Plant, Care, and Harvest Guava