What you want to do is remember the formula 3n+1. If you press F3 and read the chunk numbers you can figure it out. If algebra is not your strongest area, then take the chunk number, subtract 1, and if the result is divisible by 3, it's the center chunk
For example:
Say that the X chunk you're in is (10) and the Z chunk is (4) - that would be a center chunk of an ore so you can dig down there. (10 - 1 = 9 and 9 / 3 = 0) (4 - 1 = 3 and 3 / 3 = 0)
X = 16 and Z = 5 would mean X is the center chunk, but Z is not, so move over 1 chunk until it's X = 16, and Z = 4
X = 9 and Z = 7 would mean X is not the center chunk, but Z is
You won't find one 100% of the time due to how the generation works, but you should find one the majority of the time. Once you've found 1 center, then you can just count 3 blocks N/S/E or W without having to calculate it again. It's still annoying and tedious though, so good luck!