What is the correct phrase for "speaks well to the"

Solution 1:

The expression 'speak well of' is idiomatic:

speak well of / for

to show that something is correct or effective

The lower crime rate speaks well for the police department’s new strategy.

{Macmillan}

Of course, this has the alternative meaning 'praise'.

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Alternatively, 'testify well to' may be used:

The difficulties Leibniz and Varignon found in giving an analytic form to dynamics testify well to the fact that their effort was not trivial.

{The Development of Newtonian Calculus in Britain, 1700-1800}

testify to

testify 2 to provide evidence that something exists or is true

testify to: These ruins testify to the existence of Roman occupation.

{Macmillan}