You can try this tool: https://adwords.google.com/select/TrafficEstimatorSandbox But in my experience, it's not the most accurate.
You can have a vague idea with adwords tool, but really only to see if one topic has better cpc than another. More precise I have no idea because adsense prevent us to disclose our stats on forums, otherwise we could share each others stats and get a general idea, better than the adword tool. It could be done, but anonymously, and on DP forum we're not anonymous to google since we have signatures, this is the easiest way to find out if they want, but I'm sure they have many other ways with cookies and whatnot.. Their ways are beyond our individual understanding capability since google is a massive team of big brains and large network of computers, even one genius with IQ 160 can't compete fairly against thousands of other genius working at google. And my IQ is not 160