There is no easy way to answer this question, unfortunately. Silent letters in English are a complicated topic, because there is no single explanation for all of them.
Take the English word "knight," for instance, in which the k is silent. It is derived from medieval Dutch or German, in which the initial k was pronounced (as it was, apparently, for some time even in English). Hundreds of years later, we have simply stopped saying that initial k, but the spelling remains the same.
With final e's, as in "knife" or "safe," the placement of that e usually indicates the use of a long vowel in the pronunciation of that word. But again, this has to do with changes in the way the words have been written and spoken from Old to Middle to Modern English.
In time, you should get better at recognizing which letters are sometimes silent, and which are not. K's and g's before n are almost always silent, for instance, as in knife or gnat. But almost every letter (including vowels) can be silent ... that's why it's so important to read constantly to improve your vocabulary, and make sure that you always have a dictionary at hand! (Not a phone.)