You can look at the number of subscribers.
All things being equal, a larger list should offer a higher response than a smaller list.
But all things are rarely equal.
Sometimes large lists are large because they have inflated (fictitious) counts, or because the list owner leaves expired or old emails in the list, and continues to report them as subscribers, even though they never open their emails, or consistently bounce.
You can look at the price of the ad, although this should be done in comparison with the wait time and the number of subscribers on the list.
In general, an ad that costs $500 will probably have more of a response than one that is priced at $50.
Again, if the wait list is long for the $500 ad, then it can possibly be assumed that the advertisers are willing to stand in line and possibly pay in advance to mail to that list.
If the ad is less expensive and available immediately, that should be a warning sign.
So how do you write an effective ezine ad? When you are first starting to run ezine ads, you should generally start with classified ads.
Generally, if the newsletter will not convert readers into your own subscribers with small classifieds at a reasonable rate, compared to the cost, then it will not convert readers well with a full page solo ad.
Sure, you might get more subscribers with a solo ad, but not more in comparison to the cost of the ad.