Like most cryptic indicators, grammar and sense dictate where they should appear in relation to their fodder (the part of the clue the indicator works on).For example, an anagram indicator I often like to use (because it's versatile for the surface reading of the clue) is 'doctor' i.e. to doctor a document = change it. However, 'doctor' must always precede the fodder. "Doctor [fodder] for [definition]" clearly indicates we must doctor (find an anagram of) [fodder] for a synonym of [definition], whereas "[fodder] doctor for [definition]" does not.All that is to say, it is important to parse the clue and ask yourself what it's telling you to do literally. Below is a link to an extensive list of anagram indicators.