Initially, Polo's fellow Europeans doubted his tales of unbelievable Chinese innovations like black rocks that burned (coal) or paper money that could be exchanged for gold or any kind of product. To be fair, paper currency is a strange idea - but much more convenient than carrying around huge slabs of gold or salt to pay for things!
More recent skeptics have focused on what they perceive as gaps in Marco Polo's otherwise keen observations of China. For example, he never mentions women with bound feet. However, the Venetian spent the vast majority of his time in the Mongol court, where women would not have had Han Chinese-style "lotus feet."
Another example is the Great Wall of China, which also receives no notice at all in Marco Polo's story. However, the Wall was in considerable disrepair at this point in history, and the Mongols had no interest in reinforcing it - after all, it was built to keep people like them out of China! Kublai Khan would have been terribly inconvenienced by a continuous wall dividing his Mongolian and Chinese territories. In fact, it wasn't until the ethnic-Han Ming Dynasty defeated the Mongol Yuan rulers that the current unified Great Wall went up... to keep the Mongols out.
In the end, there's no way to definitively prove whether or not Marco Polo went to China. For my part, though, I believe that he did.
Drawing of a young Marco Polo from Stock Montage / Getty Images.