Antivirus maker Sophos put up some basic guidance that can help answer that perennial question. The tips, such as looking out for changes to the default search engine or random site redirects, are geared towards the non-expert home user.
I took particular notice of the warning to watch out for getting snared by a fake antivirus site or warning if you hop online for advice on determining whether you're infected; such scareware scams are more popular than ever among online thieves out to separate you from your cash in any way possible. And it would only add insult to injury to end up with a rogue antivirus app on your system in addition to potential malware.
The tips end with the common advice to use known online antivirus scanners if you suspect a problem (again avoiding fake scareware), since malware will often attempt to disable existing antivirus programs if it managed to infect a PC before that program could detect the baddie. But since Sophos only lists its own free online scans, here are some other options: