You try. You start with let's say two and compare them - check which one performs better in terms of CTR and/or conversion rate. Then you write other ads, test them and stick to one(s) which give the best results.
Write a good headline to get your readers Attention.Create Interest. After you've captured attention, Create Desire. Once they are interested then Tell them what Action you want them to take
Split testing is definitely very beneficial in the long-term. I generally make one adjustment per day and stick to what works best during a particular campaign.
Ads that give high click through rate, usually the title is same with the keywords that customer search for . That is why most of people put keywords default in ads title so it appear the same keywords as the searcher type in google search. 1st line description put promo and benefit Use powerful call to action in the second row of description .
There are a lot of helpful tips about how to write an effective ad. But here is what I learn, you need to set up or establish a problem, propose a solution (product you are selling) and require them to act right away (buy your product) if not they will loose the opportunity. Hopes this helps.