After you begin to get their communication, you can check them and understand which ones are worth advertising with and which ones you can forget, at least for the moment. To decide whether an ezine is a good pick, check to be certain its internet page is not on a free platform like Blogger or WordPress. Make sure that they obviously state what they offer vis advertising.
As far as the mails you'll be getting from each ezine, first make sure that you must opt-in two times in order to receive the newsletter. This ensures the subscriber base is respondent. Also, check to see the publisher informs you of the types of email addresses that they can accept. Many e-mail suppliers block spam.
If the ezine will not let you use a few of these, this is a nice thing. It suggests that mails will be delivered to recipients rather than to the junk box. If you are receiving a number of email per day from the ezine, you do not need to advertise with them. That's too much competition. And you only wish to go with the ones that have "real" subject lines supplied by the promoter and not by the publisher. You would like to be able to build your own subject line for your advertisements. Try the content of the newsletters you are receiving. If it is valuable content and not just ads, then it's worth going for.
You may have more than one ezine advertising service that has passed your "test". At that point, just go with the least dear one.