The boundaries are definitely difficult to classify because flowering to fruiting is more of a continuum, and some taxa are more challenging than others. It also complicates things because you could have a plant with fruits and flowers present at the same time, with certain flowers being farther along in the continuum than others. In fact, the tomato plants in my yard are in this state currently.
These standards are not officially applied to iNat, but there is a Plant Phenology Ontology, which has specific definitions for different phenological states. A lot of these definitions were based on botanical definitions and/or practices used by the National Ecological Observatory Network, the National Phenology Network, or other global monitoring networks.
Unfortunately, looking through ontological definitions isn’t super user-friendly if you’re new to ontologies. But you can look through definitions and terms in the PPO here:
Here is the definition of ‘fruit’ according to the ontology: https://www.ebi.ac.uk/ols/ontologies/ppo/terms?iri=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FPO_0009001
It is interesting to note for your milkweed question that the ontology does separate the concepts of fruits and seeds.
I wanted to also put the links to ‘seed’ and ‘fruit presence’, but iNat apparently won’t let me use so many links cause I’m new. Oh well!