The easy answer to the question is that you don't need to -- you can simply assume that you are, all the time. Even your home network is constantly being bombarded with attempts to break in, but the good news is that the vast majority of these attempts are foiled because the vulnerability that was being targeted has been patched, or because security measures such as firewalls stop them.
What's more interesting is to ask how you tell when you've been successfully attacked. I've talked in a previous posting about the term "cyber attack" and why I don't particularly like it -- because in many cases it's very difficult to tell whether you've been attacked. That's because the attacker's complete objective is to avoid being detected -- and in many cases they're very good at it.
By and large, the best answer is that you should suspect you might have been successfully attacked if things start behaving strangely. In some cases, you might spot that by good luck -- it's surprising how many cases have been detected because someone complained about their PC going slowly.
But rather than trusting to luck, we think you're better advised to put measures in place to look for -- and investigate -- strange behaviour. Of course, that's not an easy thing to do -- only if they make some fairly serious mistakes will an attacker reveal themselves to the user of the affected PC. But increasingly effective technology solutions are now coming onto the market, and we believe they're rapidly becoming indispensable for anyone who really cares about security.