Is there a Japanese idiom equivalent to "take with a grain of salt"?
In other words, an idiom which means "don't accept this information as completely true or the complete truth".
This may be close to that phrase:
話半分 denotes where roughly half of what is said is a truth, and half is a falsehood or exaggeration according to Daijirin.
Another similar phrase may be
割り引いて聞く, which means something similar to "discount some of what somebody says".
See also Space ALC for more expressions.
This literally means "don't swallow it whole" (like a pelican), in other words take it with a grain of salt. For example, when I went clothes shopping recently in Japan and the staff kept saying I looked so good in various things I tried on so I ended up buying a bunch of stuff. My Japanese friend later rebuked me by saying
"The staff are salespeople after all, so take what they say with a grain of salt." 奴ら here adds a mild derogatory connotation, like "slimy salespeople" but not as strong.
Other ways partly mentioned by others are also correct:
Ｘさんの言｛い｝うことを割｛わ｝り引｛び｝いて聞｛き｝く (or 話｛はなし｝半分｛はんぶん｝に聞｛き｝く)