Look at a recent auction or two if possible. If the seller seemed to get a really high price and there is one "bidder" who kept pushing the price up but has single digit transactions, that might be a sign of shill bidding.
I had this experience a couple months ago. maybe the bidder was a shill but then again, maybe not. I had 3 different auctions in my watchlist for a Winchester 9422 and I saw the same bidder bidding on all three auctions. He was definitely driving the price up from what I could tell. just seemed curious to me that this one guy was bidding on multiple 9422's being listed. Either he's trying to buy up the remaining 9422 stock on the market or he's shilling for someone. Needless to say, it really left me PO'd. I wound up going through gunaction.com and found a 9422 at a fair BIN price and nabbed it.