Both acoustic testing, via tapping or Chladni patterns, and deflection testing can work well. Both have strengths and weaknesses. I've used Chladni testing for years, in part because I already had the setup. It's not the quickest way, but it give you more information: the Young's modulus along and across the grain, with the associated damping factors, and density (which you need to measure to get the others). One advantage of Chladni patterns over tapping is that it alerts you to when there might be confounding factors, through the mode shapes. Tapping doesn't get you that information, but is quicker.
Deflection testing as Jim shows, using 'free-free supported' plates, can also be accurate. Some folks advocate clamping one end and measuring the deflection of the free end, but there's a risk: if the clamp is too loose you get a low reading, and if it's too tight you crush the wood which can also throw the reading off. From what I understand, with deflection it's best to take two readings, since you can get slightly different ones depending on the grain of the wood.
I'd advise setting things up so that you can calculate the Young's modulus of the wood, rather than simply getting a deflection alone. Young's modulus is a standard engineering measure, and can be used to calculate the 'proper' thickness of the top. It's also handy to be able to compare your results with those of other makers: rather than having to re-invent the wheel you can talk to other makers who've worked it all out. It can shorten the learning curve a lot.