Spotting a single shilling post is next to impossible unless it is just a crap answer. Of course if it is a crap answer there are various responses depending on how it sucks. If a little bit of care is taken by the potential shiller though it will be an ok answer.
Honestly I have to say I'm not very worried about shilling posts because:
- This site is specifically for recommendations for products (commercial or otherwise)
- The developer(s) should know their own product better than someone else and to some degree the field that it is in.
- If it is a crap answer for other reasons it will be removed pretty quickly
- If it is a good answer other than that the user has an undisclosed relationship to the product that is ethically questionable but does not actually harm the site.
Really the only case that it harms the site beyond being a bit more moderation (which so far I haven't seen any serious amount of), is mediocre answers that are posted to every question that isn't a great match to the question. So in the case of one mediocre answer just comment/suggest edits to improve it and unless you think there is evidence of shilling leave it at that.
A pattern of posting of similar mediocre answers to questions from one user consistently recommending one product (or company's products) is the only type of shilling that I actually care about really. That is of course only spot-able if you are monitoring the site or you check user history occasionally when you have a post that just raises flags for you for no discernible reason.
So in conclusion, the signs that a post is shilling are mainly:
- Less than perfect match to the question requirements.
- Link-only (but then just flag NAA because that needs very little reviewer time to deal with).
- Close to verbatim copying of advertising information (but then just flag VLQ).
- A pattern of the above.
- A pattern of linking to one product/site/company.