I don't know, unfortunately. It seems to me it would be hard to measure. The relative importance of human capital effects and signalling effects are also very hard to measure. On this, I would recommend reading this article on how a recent British government report completely disregards the signalling theory and without argument assumes that the human capital theory is right:

Regarding the signalling effects of higher education, BIS [Department for Business, Innovation and Skills] assumes that “higher earnings reflect higher productivity as a result of learning (ie, there is effectively no signalling effect)”.

This is an “extraordinary” assumption, according to Alison Wolf, Sir Roy Griffiths professor of public sector management at King’s College London. “I had no idea they didn’t take [signalling] into account at all,” she said, adding that failing to account for it was “bad policy and bad economics”.

Further down:

The debate between human capital and signalling theory is a political as well as economic argument, according to David Palfreyman, director of the Oxford Centre for Higher Education Policy Studies. What he calls the higher education “lobby”, through Universities UK, “naturally argues the human capital theory line as a way of getting extra public money and/or justifying the charging of high tuition fees to the punters”.

Politicians also “hope” human capital theory “might be true as they flounder around trying to find something they can do to hopefully achieve economic growth”, he said.

Asked if BIS would take signalling into account in future given the report’s findings, a spokeswoman said the study “acknowledges the potential role of signalling in driving the observed earnings returns to degrees” but also “highlights that there is no consensus from the literature about their magnitude”.

“The authors therefore recommend that we should not reduce the estimated earnings returns to reflect signalling – as any reduction we applied would be arbitrary,” she added. “We will however continue to consider the implications of signalling effects in policy development and also in the development of any research in this area.”

As if zero signalling effect is not arbitrary... This is quite outrageous actually and should have received more attention.

Possibly we shouldd have another discussion of human capital vs signalling in higher education, and what to do to minimize costly side-effects of signalling.