The studio that ghosted a $4k invoice — how do you vet employers before saying yes?

Booked what looked like a dream editorial, delivered everything on time, then the studio went dark for two months on a $4k invoice. No contract teeth, no recourse, and I didn't want to torch the relationship publicly. Posting anon for obvious reasons. What are your actual green/red flags before you commit to a new employer — deposits, kill fees, paper trail? Trying to never repeat this.