Yeah, water-in and water-out. You really want to measure the pressure drop across the block (the pump is irrelevant).
Once you have a flow measurement, you'll then be able to fully characterize the block, in terms of restriction. BTW, we tend to test at 0.5, 1.0, and 1.5 gpm, but there seems to be an increasing need to measure at 0.25 gpm; some of the latest blocks run well at low flow and high pressure.
As pHaestus put it, the analog gauges are useless. First, you have to find one with a decent resolution, then you'd have to have two of them (inlet and outlet) then calculate the difference. That of course doubles your error margin, and each unit usually starts off at ~5%, leaving you with a total error of 10% (pretty useless). Each unit measures the pressure relative to atmosphere, hence the calculation.
For the sake of convenience (because water block testing can be very involved), we use a digital unit, as pHaestus suggested. The units are available as absolute or differential (or both). We want differential. Again, for the sake of simplification, if you have the ability to log the data (which requires some equipment), it'll save you quite a bit of time.
pHaestus also made a reference to fluctuations; these are indeed induced by the pump. Bill worked his way through some fluctuations, but little of his work is documented (OC forums?!?). I'll share my work, as I get to it. These fluctuations will occur at various frequency ranges, as I remember in the 120 Hertz and below. That figure is important, as it defines a parameter when you pick your dP meter; it should have a sample rate that allows you to visualize these fluctuations. 500 samplings per second or more would be a minimum, IMO.
I'll be adapting a Honeywell unit to my rig. Let me know if you want more information. Digi-Key sells it for ~$50, as I remember.
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