There are some red flags and a combination of them is an alarm bell.
- Yes check the "bid activity % with this seller".
- Also check the sellers "other items", see if the same bidder pops up on the current items for sale.
- Lots of "bid retractions" is a warning sign.
- Lots of bidding but no actual purchases or sales is a big warning (check the number beside the username).
- A bidder who places lots of little bid increases, gently nudging the price up (looking for the high bidder's limit without overstepping it by much, or at all) may well be a shill bidder. Not necessarily but watch for it along with the other signs.
- A shill bidding account will tend to show less or no last minute bids (<1hr). Again not a sure sign, but a clue.
I was being shill bid once, i spotted it via a combination of the above clues. There were lots of little bid increases coming in. I confronted the seller directly declaring that if i saw even one more bid from that particular bidder i would immediately retract my bid and report the seller to ebay for shill bidding. Of course the reply was a denial but the shill bidding stopped. A sure sign he was guilty and didn't want to be exposed.
Since then i only ever place last minute bids.