In theory, muskets are longer than arquebuses, with a heavier bore and more of an actual buttstock. The date of the painting , 1642, is a bit later than the one Dutch reference I can find. The Dutch Regulations of 1599 specify a lontslot or matchlock musket of 57 inch length, with 48 inch barrel and a bore of .78 bore, and a lighter caliber or caliver with a 30 inch barrel of .63 bore. It appears that there was a tendency for Dutch long arms to get lighter in the 17th century, and the militiaman in front, on the left, and the one in the second rank, on the right, seem to have something shorter than a musket, so perhaps they're actually calivers, even if carried by guys called ( in the Dutch wikipedia article) musketiers.

Looks like there's a 3D version set up in Amsterdam, now.

Both muskets and calivers were used by the Dutch Republic's army and its militias. Usually people don't care much for the difference and all these weapons are referred to as 'muskets'. That makes hard to decide what's what.

Let's get this out of the way first: the Rijksmuseum states that the militiamen are armed with musketten ('muskets').

With that out of the way, let's take a closer look. Jacob de Gheyn's Exercise of Armes from 1607 shows us that calivers were loaded using a flask, whereas muskets were loaded from a bandoleer. See these two picture for a comparison: handling a caliver (notice the large flask on the right hip and the lack of bandoleer) and [handling a musket](https://upload.wikimedia.org/wikipedia/commons/d/d5/Aanwijzing_9_voor_het_hanteren_van_het_musket_-V_lont_versoeckt(Jacob_de_Gheyn,_1607).jpg) (notice the bandoleer). We can see that both the militiamen in red have bandoleers, indicating they're armed with muskets. On the other hand, De Gheyn also shows us muskets were used with rests, and there's not a rest in sight. Moreover, as /u/Bodark43 mentioned, the weapons seem a little on the small side for muskets.