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, "[definition] is concealed by [fodder]" clearly indicates that [definition] is hidden inside [fodder], whereas "[fodder] is concealed by [definition]" does not. And vice versa, "[fodder] contains [definition]" cannot work with the fodder and definition swapped.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 hidden-word indicators.
"Perspire in glorious weather (5)" -> 'in' is a hidden-word indicator, clearly stating a synonym for 'perspire' is in(side) 'glorious weather' -> SWEAT.
Sometimes, to improve the surface readability (at the expense of writing a truly great clue), setters (or at least myself) will forgo a linking word/phrase. Instead of "[definition] is found in [fodder]", where 'is' serves as the link between definition and wordplay, "[definition] part of [fodder]" may appear as in the below example:
"Stretch part of jaw I dented (5)" -> 'part of' is a hidden-word indicator. 'Stretch' is the definition. In 'jaw I dented' we find WIDEN. It would be more fair if the clue read "Stretch is part of jaw I dented", but sometimes we sacrifice perfect fairness for a better surface.
Many containment and insertion indicators can be used as hidden-word indicators too. Their validity completely depends on the literal interpretation of the clue. For example, 'absorbed by' can serve as a containment indicator: "Abundance of oxygen absorbed by lad (4)" -> oxygen = O, lad absorbing O becomes LOAD.
But if you imagine it serving as a hidden-word indicator, i.e. "[definition] absorbed by [fodder]", it makes less sense as it implies a required action rather than a state of matter. "[definition] held by [fodder]" is great, but "[definition] thrown into [fodder]" implies that [definition] is part of the wordplay, that we need to put [definition] inside [fodder] to solve the clue.