Get into some meaningful reading — if you have made up your mind about guest posting. Reading different posts on different blogs (of your niche/industry) will give you an idea which of the them are better. You can use Google search to reach a list of blogs that belong to your industry or subject. Another thing that you will notice is that what kind of posts and articles get published on these blogs. Try to assess what it was that gripped your attention and you started reading that blog/post.
Once you start reading different blogs in your area of interest, you will soon be able to tell the difference between good and average. Make a list of blogs you think you should guest-post on. A little Google search will get you plenty of resources.
Contact blog administration/site owners
After you have chosen blogs you want to write for, you should now offer them your writing services. You can contact blog administration via email, phone or even social media networking. In most cases you may not be able to find their phone numbers or email, but social media is the best tool to getting in touch of them.
How to ask?
You can tell them that you want to write for their blog and that you have written for so many blogs. Mention the names of the blogs you have worked with or are still working for. It will help them assess your worth and they might have a look at your work to make sure they are getting the right man.