How do we know cyclopentadienylmolybdenum tricarbonyl occurs as a dimer? Cyclopentadienylmolybdenum tricarbonyl, $\ce{MoC8H5O3}$, dimerizes to form the cyclopentadienylmolybednum tricarbonyl dimer, $\ce{(MoC8H5O3)2}$.

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How was it discovered that it does this?

If you had an sample of a pure substance, but didn't know if it had dimerized or polymerized, what information would you need to find out if it had?
The suggested answers from the textbook are to look at the output of a mass spectrometer, or to observe the macroscopic crystal structure.

However, NMR spectroscopy is also a perfectly acceptable way to go about it.