I've had this happen as well. When it does, I go to the bid history and hit the link associated with the account that out-bid me. When you do, you will see something like this:
Screenshot 2022-06-21 061219.jpg
The 30 day summary is the key. If they have a fair number of bids over that time and a high percentage are with that particular seller, it would seem a good indicator that there is some shilling going on. It isn't a perfect rule, as there situations it may be innocuous. For example, if you are looking at auctions with a reputable, and large volume seller, like Greg Morris it is probably coincidence as many buyers (like me) only bid on auctions with those sellers. But, if it is a marginal seller and I see a suspicious history, I walk away.
Note: The picture above isn't what I would consider suspicious. I only used it for illustrative purposes.