1. You notice that you pay more than usual for your mobile phone bill
This is a sign that some trojan might send SMSs or make phone calls to super expensive phone numbers, sometimes even oversees. The problem with these calls is that it is very hard to prove that you didn’t manually and intentionally made them. Most of the time the mobile phone operator will ask you to pay first and then explain later.
2. Data usage increase
Malware usually sends back your private data to the cybercriminals that created it. If you notice an increase in the data usage or if your provider is slowing down your data transfer because you consumed too much in a month, it might be a sign that malicious software communicates without your knowledge.
3. Calls are interrupted often and SMSs don’t reach their destination
Even if you see that you have maximum reception sometimes the most basic functions of the phone don’t work reliably. Sometimes malware tries to intercept the calls and even re-route them to more expensive numbers or through proxies.
4. Battery consumption grows unexpectedly
If without using your phone more than usual you notice that the battery drains, there might be some program that is residing in the active memory. Such programs can be trojans that try to intercept the calls and SMSs you make.
5. Bad overall performance of the smartphone
If your smartphone becomes slower than usual and apps take much longer to start and function, something might be using the CPU and the memory of the phone. Review the last apps you installed and try to uninstall them to check whether one of them is consuming the resources. However, note that this might not solve your problem if you installed a malicious app. Most of the malicious apps install backdoors in your device and will download additional payload without you noticing.
6. Apps crash unexpectedly
If apps that usually worked without problems and didn’t get updated lately, suddenly start to crash, might be a sign that something is interfering with their functionality. It could also be that your smartphone doesn’t have anymore resources to run the app because something else is using it (see 5).
credit goes to http://techblog.avira.com/2013/04/29/is-your-smartphone-infected/en/