iPhone: How to tell if Phone is in Silent Mode

There is no way to tell. Even when silent mode is on, iPod will play through the speakers, so even the built-in apps don't know the difference. I am curious to know how they detect a plug/unplug from the headphone jack to stop the music though.

Did you find an answer ? I'd really like to know if there is an api that would allow us to query if the phone is in silent mode or not....

any help appreciated.

That would probably explain why there is no icon in the top menu bar that shows the status of the switch.
One have to look at the switch on the side of the phone to figure out if it is in silent mode or not.
Somebody slept on the switch on this. ⚠

True,
Just today I put my iPhone in silent mode by mistake, several calls came and went while I was busy working, then I saw missed calls and voice messages.

Apple really forgot to put the icon out there !

MUCH agreed.

I had an app on my Treo ("Show Off") -- http://www.hobbyistsoftware.com/show-off-more.php -- that remedied this issue. That app changed the background color across multiple apps...so you always knew whether your ringer was on or off. Another useful quality of this "background switching" was that I often had my ringer off at night before going to sleep, in a theatre, etc. -- all environments in which I actually preferred a dark background in order to spare those near me. So my "silent" theme was white text on a black background; and my "ringer on" was black text on a white background.

This kind of "theme switching" would be decidedly un-iPhone-like, but man, I loved that Treo app...!