How do you detect duplicate content issues?

I'm just curious there are lots of suggestions on what to do afterwards, but how do you notice that a site has a duplicate content issue?

As we knew that Google doesn't like duplicate content although it is not a Google penalty but it can confuse Google in what you want to rank first.

What do you guys suggest me?

Make sure you are using canonical tags on your pages, and you really do not have to worry much about duplicate content on your site(s).

SEOPub said:

Nofollow will not have an impact on duplicate content.

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Indeed that makes sense... From one of your previous posts it was useful to also learn that nofollow gobbles up a proportion of link juice to a black hole so there really isn't a reason to use nofollow as means to sculpt link juice on a site unless the intent is to mask affiliate links in which case nofollow/noindex is the way to go... If nofollow meant other links gained greater link juice then it maybe worth thinking about... But alas that is no longer the case ;-) Nice one for spotting that!

SEOPub said:

I would get rid of tags and author archives on 99% of sites just to avoid that altogether. Tags are a waste of linkjuice in almost all cases. Same with author archives.

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These days I'm removing post meta data entirely e.g. author, category, post date etc (below title) as well as post archives sorted on date (sidebar)... Its also tempting to get rid of categories too (sidebar) and just focus on silos as means to funnel link juice and use that as means to provide some form of user navigation in replacement to categories but that might be stretching it a bit with impacting user navigation.... On that specific point i've been thinking about how to implement that dynamically in relation to which post is displayed...