There are a number of indicators that can be used as an "indication" that a share could be ripe for a short. However they cannot be used in isolation. Most traders have their own "system", i.e. a "list" of indicators or patterns that must all fall into place before considering a short. These could include RSI and/or MACD reflecting an overbought situation, share price movement, volume traded, industry sector sentiment (e.g the last few months had been great to short most Retail stocks), Double top and Head and Shoulder patterns, Doji candles etc, etc. - the list goes on. and just to complicate matters, not all shares follow the identical "criteria", hence a trader should have different systems for each shall we say "group" of shares (e.g. NASPERS behaves very differently to Anglo under identical indicators). And even once all indicators are in line, it still does not mean that the share price will drop. That's when the trader's pre determined risk management and strategy come into play. Is the trade worth the risk?
In my experience, shorts generally run for a shorter period compared to longs - but it's not a rule - the story of "Bulls take the stairs while Bears jump out the window" comes to mind.
