Actually, you can calculate and estimate it fairly accurately in all scenarios. You can probably find an old flow rate estimator speadsheet of mine I made a while back.

I had a spreadsheet developed that would add up the restriction curves, add up the pump curves and solve for flow rate. Unfortunately its pretty outdated anymore.

I became interested from questions like this post and wanted to understand it better. Bottom line is that pump performance varies depending on flow rate. The only way to capture the full performance of a pump is to compare the entire curve and its very possible to have two different pumps perform better each in their own respective areas.

Reality is though that even if you can measure a flow rate difference, it doesn't mean much in terms of temperature...and it means even less in terms of overclock.