Pasting my response from another forum where this question was also posted:

PEG, polyethylene glycol, is water soluble. The number after the PEG denotes how many repeat units are in the chain (how many units of ethylene glycol). The higher the number, the better the water solubility generally speaking. So when this is reacted with the end group of a silicone and has a sufficiently high number, then the new molecule will be somewhat water soluble. This is also dependent upon the size of the silicone, so it is a ratio of both. Say, if there were 5000 repeat units of dimethylsiloxane (the building block of dimethicone) and only 8 units of PEG, I would have to guess that this would be not very water soluble. In contrast, were there 200 units of PEG and 1000 units of dimethylsiloxane, then perhaps this would be somewhat soluble.

There is no rule requiring disclosure of the size of the silicone though, so we really have lttle way of knowing, so I generally look for as high a PEG-number as possible. Of course, I also use shampoo once a week, because I think it is necessary due to so many non-water soluble or sparingly-water soluble things being in most products we use.