In some siddurim that are printed in certain Hebrew fonts, you will find words such as משה (Moshe) or חשך (darkness) that do not print with two nekudot. The most explicit way to point these words would be to have a holam over the first consonant and a "shin dot" (what's its real name?) over the shin.
Instead, the siddurim I am referring to only put the holam in, not the second dot. In the two examples I give, the ש is indeed supposed to be pronounced as a "shin", but I do not know if this is safe to always assume this. Are there cases where a ש after a holam should actually be pronounced as a "sin"? How would you recognize those cases, a grammatical rule or just knowing the word?