One gauge that most people start with is the number of followers you have. Well that does let you know how many have seen your blog and liked it. However it doesn't give a real idea of who is reading because, often people don't check their reader.The same holds true if you use tools like networked blogs or feed burner or any other reader.
Another more reliable method is to check the number of visits your blog is getting. Google Analytics works really well for this but Blogger Stats works really well too. I didn't notice it until recently. When you go to blog page, there will be a tag for Stats, it right there but sometimes if you're not looking you won't see it. You can get an overview for right now, up to all time. It breaks down visits to by posts as well as to the whole site, by visitor and even has an easy tool to remove your visits from the data.
You can use this to see what posts are people reading, from this you can see what type of posts are getting the most hits. Type of browser, location, even what OS they are using and of course, where this traffic is coming from as well as search terms. I'll admit it only shows the top 10 of each but it helps out.
Comments are a big clue that not only are people reading but they are interested enough in what you say to say something back. Now you have it on something. You have readers and you have excited them enough to respond.
the tools can give you a feel on how you're doing.
Although there is still one gray zone, RSS feeds and subscriptions. RSS feeds and subscriptions are probably the easiest ways to read a blog, I know if I really want to keep track of a blog, I have it go straight to my email and read it there. I get every post and don't miss anything. Sadly though there is no way that I know of to track if or how many people are reading this way.