Looking for a solid 3pl warehouse in the los angeles area for FBA prep, how do you vet these places before committing?

I've been doing my own FBA prep out of a garage rental in the inland empire and it worked fine at low volume but I'm scaling up and it doesn't make sense anymore. Spending entire weekends labeling, polybagging, and building carton shipments when I should be focused on sourcing and managing PPC.

Started looking into 3pl warehouse options in the los angeles area since that's where my supplier ships everything and I don't want to add unnecessary transit to the equation. But there's a ton of prep centers out here and they all kind of say the same stuff on their websites. Fast turnaround, competitive pricing, blah blah.

What I actually want to know is how do you figure out which ones are legit before you hand over inventory? Like what questions should I be asking, what red flags should I watch for? I've heard horror stories about prep centers losing inventory, mislabeling stuff so it goes to the wrong ASIN, or just sitting on your product for weeks without communication.

Anyone in the LA area have experience finding a prep center that actually delivers? How'd you narrow it down?