Marketers are going to be good self-promoters (it is their job, after all), and the good ones are worth a lot because they can save multiples of their comp in wasted spend, but most are unfortunately flunkies who parrot Google's talking points, always use automated bidding and constantly change KPIs.
Either way: you're just a few clicks & bucks away to test your ability. Especially in this space.
I locked myself out of a girls flat. She was hungry & freaking out, though i knew i was almost getting in, if i just had strong metal to slide-open the bolt. But stores were closed and i couldn't steel a spokes of a bike. So we called a locksmith. I told him i know we're being ripped off, but i am happily paying if he explains to me what he is doing and what alternatives there are. He opened the door in a few seconds and i paid 100 bucks. Inside i immediately ordered a lockpicking set & started to practice. I am not good at picking locks. But now i at least know when it's feasible to try myself and in which cases i should get ready to be ripped off again.
The ad space is full of people waiting to BS your money out of your pocket. Find out when it's ok to take BS without it leading to BS results – and when the BS predicts a BS outcome.
Or: Trust your gut. It's always right. And if it's not, try again & pretend it's never been wrong!
Otherwise they can just unfortunately obscure performance and the way they manage their accounts won't be clear.
There's some good advice in this thread but some of the specifics aren't consistent with current best practice.
Context: 8 years PPC experience