Warning Signs if need Spotify and Apple Podcast Subscription Promotion

[chasingwhimsy]

YouTube has a fantastic Promotion feature and has some real value. In the past I have not had luck when looking got Spotify and Apple Podcast Promotion. 

I found many sellers use very similar language about "Engage Listener" and "Organic Subs". Although some the numbers increase it's only to the 1 or 2 recent episodes and any subs are not continuous listeners week to week. 

What are things I am looking for when shopping for Podcast Promotion? 

What things are warning signs that I should see and stay away from?

What are appropriate questions to ask before agreeing to any gig?