It will never be:
At the beginning of a word
right
After the prefix (though the hard sign might be)
I think, right
It will always be at the end of:
An infinitive ending with -т
right
A reflexive ending with -с
right
A second person verb ending with -ш
right, BUT in this case you can't hear it: "шь" is pronounced like "ш".
If it wants to, it can appear:
At the end of a masc/fem noun (particularly ones ending in -т -л -н -с or -р)
At the end of a masc. it never appears (there are only few exclusion: тюль, туннель, шампунь)
In the middle of a word, between two consonants (look for -т -л -н -с or -р followed by a nice hard consonant like к).
Am I on the right lines?