I have a particular talent for spotting negative positions in stocks and trends in general. I'm not trying to prove that I do, take my word for whatever it's worth to you. In equities I've derived it from the Graham-Dodd-Buffett approach. I've found it exceptionally easy to spot when to short stocks by reversing the Graham Dodd approach (it works for spotting undervalued equities, and it works equally for spotting overvalued equities, I just happen to be good at that side of it).
On the technology side, if you understand Apple's business model, and you understand how technology markets work toward consolidation around standards, and then you apply the context of what Android is and its vendors / support / ecosystem (and whether it's of high enough quality to be good enough for that 80% to 90% of consumers, to rule ala Windows) - the rest is obvious, it lays out a momentum line that can't be stopped.
There are exceptionally painful consequences to Apple losing market share that very few are pricing into things. The mistake I see often is linear thinking on market share to profits; eg if Apple is making $30 billion on 20% market share, said people then think Apple could make $15 billion on 10% market share (when in fact there are numerous destructive penalties for falling below thresholds in the market, regarding developers / apps, music, media, consumer perception, negotiating position with suppliers, stock price falling and losing key employees and on and on).