> I meant, how would you test the veracity of the statement 
> "because their ancestors did so," since we have no access 
> to that time period.

There's an incredible amount of access to that time period, the fossil
record is very complete and excruciatingly well-documented on angiosperms.

The fossils tie present flowers to some of the past flowers, and can show
the development of specific technical "enhancements" in specific lines of
plants, such as the "closed carpel", and the appearance and disappearance of
sepals, petals, and stamens from any one flower type. 

For insects, visually-attractive "nectaries" developed, the ovules started
to "clump" closer together, male (pollen-carrying) structures developed, all
clear tangible changes that optimized for insect pollination. 

The closed carpel in a showy flower that we see today can be viewed in
perspective as part of a slow progression that ensured outcrossing by insect
and animal pollinators.
The closed carpel "guarantees" that outcrossing happens. 

Outcrossing is the big win for the plants, as there lies long term species
survival.

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