There are many ways to estimate the amount of traffic a particular site/ blog receives.
1.This is one of the best tool you will ever need to find out entire history of site. SemRush offers another paid service that's useful for researching the traffic levels to competing websites. You can also use this service for finding information about your competitors' paid ad campaigns and other details. It even displays all the terms for which that particular site is ranking. The free version is enough to get the traffic estimates, however to see search terms, backlinks and other things, you need to have paid account.
2.Alexa tracks website visitor traffic rankings for many websites on the Internet. Their data is not completely accurate, because it involves flawed methodology. Alexa ranks websites based on how much traffic they get from users who have chosen to install the Alexa toolbar. Alexa toolbar users are a small minority of website users, which makes the data somewhat skewed. However, basic Alexa data is free to the public and easy to obtain, and it does give you insights you wouldn't have had otherwise.
3.If a website is using the BuySellAds platform for generating advertising revenue, the site's ad impressions are disclosed in the BuySellAds marketplace. These are accurate and analytic verified impressions, so much accurate than Alexa.
4.Advertise/ Press kits: Many websites make their metrics available to potential advertisers via press kits, media kits or "advertise here" pages. These pages typically might include data about the number of unique visitors, repeat visitors, monthly pageviews, daily pageviews, newsletter subscriber counts, rss feed subscribers and social media followers the website gets.
5.TrafficEstimate: This free resource gives you a bunch of data in one place, although some of it is inaccurate. You'll be able to find out some basic Alexa rankings, keyword phrases the website is targeting, other websites targeting similar keyword phrases and websites with close relationships. If you're researching smaller or newer websites, you might not get any results from this tool. The closely related websites report is flawed; in some cases it does find closely related sites, but in other cases the sites it shows are all unrelated.
Source:quora