We can say "in Turkish alphabet "ğ" is pronounced as غ in Arabic. Shortly we can say ğ = غ This is writing language and sometimes we read it as also daily speaking ğ = غ But sometimes Turks (we) use it so fluent in daily speaking... In Arabic you must read and use always غ by throat... But in Turkish, sometimes (in speaking language, but not writing) you can use it so fluent so you don't have to emphasize on "ğ"... This is the difference between Arabic and Turkish ğ and غ in daily speaking... Your question is this: "How should I know if it is silent or not in a word?" But there is no problem about ğ silent or not silent... We can't categorize it as "silent ğ" or "not silent ğ"... Just don't try to emphasize it by throat in daily Turkish speaking as Arabic... That's the difference of them...

ğ is never pronounced as غ in Arabic. It just double the sound of the vowels around it. for example "ağaç" is pronounced as "aaaç". That's it. Hope it helps =))