I don't care about social. I don't care about tweets. I only care about email.
That being the case, I'd do the following:
1. Ask for the site for the sub url for the "daily newsletter."
2. Sub.
3. Watch the newsletter for two weeks (to get an idea of engagement for each day of the week).
4. Each day, check the sales ranks of the books in your genre that were promoted in the "daily newsletter" the previous day.
It's not a perfect approach. If a promoted book shows a sales rank of 1,234, you won't know whether another promo contributed to that rank. But this is as close as you'll get to knowing whether a promo service is worth using.
If a promoted book ranks 50,000 the day after, you can ghost the service knowing you're not missing anything.