Windows used to be the playground of malware and viruses and it still is but as more and more users migrate over to mobile, the spread of bad apps and data stealing malware have spread especially on Android. There is no denying that.
Here is a sort of a beginner's post from Read Write regarding virus and malware on your mobile and signs that your device might be infected. While the post is directed at Android devices, make no mistake, it's about all mobile including iPhone, Windows Phone, and other platforms as well.
First, your battery life. True if the malware is doing stuff in the background. The post talked about displaying ads. I think worse things can and probably are happening.
Then there's dropped calls. I'm not so sure about this one. While the post hesitates to blame the carrier, I don't have that issue. However, the rest of the post are more interesting. Slow performances, higher than usual phone bills, and data spikes.
Now, keep in mind that none of these alone could be attributed to malware. Battery life on any device is pretty bad in general even a pristine Android install like that on a Nexus 4. And yes, while Apple has done wonders with the battery life of the thin iPhone 5, it's still "meh" to me.
But you get bad battery life, lots of data, slow performance, and you getting charged for things you did buy or services you don't have, that's usually a good sign your device is infected.