How to Determine if a SEO Specialist is Good

Disclaimer: This is not a work offer, I am not looking to hire, just want to understand how to properly evaluate whether someone knows what they are doing despite not knowing too much about the industry myself. I want to look for someone to help with my website's SEO. But I am not sure how to go about finding the right one. Hoping to get some help here: 1. does the niche matters? If an SEO specialist is used to working for niche unrelated to mine, would that affect their work performance? 2. I've been browsing upwork and fiverr, but I wonder are there better platform to find someone? Or generally these platform are the ones to go to. 3. What sort of budget should I be expecting for a website just starting out with zero traffic? Not looking to spend tens of thousands of dollars just yet, but also would like to invest into the site. Appreciate any help and advice, thanks!