How to find out if you're being DOSed?

What's an easy way to find out if your site is being DOSed? My bandwidth has suddently increased by 4 times daily for the past 7 days, but by looking at the logs it seems most of the traffic is to my main index.html page. That's why it doesnt look like a hack, just a DOS perhaps.

Thanks for any help.

 

 

 

 

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