What is the best method of testing Google UAC creatives in a pre-Ad Groups setup?

I have found that using the process of elimination tends to work well for UAC creative testing. Start off with the creatives you feel will work best based on historical performance/crossreferenced with other networks.

Depending on your KPIs and volume you can expect to get an understanding of what works in a couple of days or a week. I would be hesitant to let it go longer without intervention.

You can check out creative performance from the UI and segment by your goal event(s). I tend to export everything to a sheet to apply my own formula / mess with numbers as needed. In general you will see a couple of creatives spending the majority of your budget.

I usually go for one of the following options:

1. These are your best/worst performers. You can conclude your testing and remove underperformers. Your campaign is now optimized. Let the campaign run for a week or so and add in some new creatives and repeat the process.

   You will now cycle the campaign between experimentation and applying learnings continuously this way. This helps to stay on top of fatigue and help to understand -- and hopefully predict -- your creative half-life.
    

2. The testing has only begun! Regardless of how these top spenders perform, remove them and find the next top spenders after a couple of days. Keep  cutting down creatives until you barely have any left. Compare them to each other and determine what your top performers are.

End the campaign and start a new one with your winners. It is important to start a new campaign so you don't carry over any weird learnings from the experiment.