l don't think it's far-fetched to say that most people have no idea if their mobile devices have been infected with malware, simply because they don't think mobile malware even exists (call it the Mac syndrome?).
On a PC the signs are pretty obvious. Your computer slows to a near-screeching halt, your browser re-directs you to random websites, your friends are suddenly calling asking about your career change to become a Viagra distributor (since your email has probably been hacked). Your IT guy can often tell by looking at your process names, as malware authors might name their malicious process 'svchsot.exe' to look like a legit one 'svhost.exe' (see what I did there?).
Harder to tell on a phone
According to Kaspersky malware researcher Tim Armstrong, users usually don't discover something's wrong until they look at their phone bills and don't recognise the numbers of text message recipients.
Premium rate SMS Trojans are the most common type of mobile malware. This malware disguises itself in a legit-looking app, and secretly sends SMS short codes that bill the caller.
Nor will an average user really be able to tell by checking app permissions. Android developers can choose from dozens of permissions, and as Armstrong notes, it's often impossible to guess which are legitimate and which are warning signs.
Another sign, according to Lookout Mobile, is sudden, decreased battery life.
Beyond malware, however, your mobile device might be subject to an even greater threat: an aggressive ad network. Google Play doesn't weed out apps with such networks (they own one of the most ubiquitous ones, AdMob), but many vendors now have ad network detectors. Check out Lookout Ad Network Detector or TrustGo Ad Detector.
No excuse for no antivirus
The only way to confirm your suspicions is to install an antivirus app. There are lots of free, fast, non-intrusive mobile antivirus solutions out there. Lookout for Android is our Editors' Choice for Android security, but other high-performing malware detectors include F-Secure Mobile Security 7.6 and McAfee Mobile Security 2.0. All have free versions that include a quick malware scan.
Theoretically these tips apply to the iPhone as well, but at the moment the closed nature of the Apple App Store means there are few examples of known malware plaguing iOS. Most iOS malware is distributed as fake rooting software.