You have to look at each case individually. Here’s a guide I wrote about it back in 2016: www.elitefourum.com/t/how-to-recognize-shill-bidding-on-ebay/14004/1
These days sellers are getting better at it and using things like private listings or high-feedback burner accounts. The days of 0-feedback 100% bid activity are likely over but if you do some digging it can become pretty clear who is shill bidding.