You’ll recognise bad links if they come from a country where your main audience is not located (you are in the US and you have a bunch of random links from Albania? Not a good sign!). Maybe the link is hidden in a site that seems like the text was scraped right off another site (maybe even duplicated word for word). If the anchor text of the sites linking to you is ‘too optimised’ (for exact keywords you want to rank for, instead of a more natural mention of your site or business name).
They won’t forever harm your page but you should take steps to eliminate them. You can find the contact details of the website owner and ask them politely to remove the link. If no one responds, or if you are asked to pay, disavow the unwanted links that can’t be removed (this means that Google will not take these links into account when evaluating your website).
In a last case scenario, you could get rid of the entire page that is being linked to, and we’ve known some to get rid of the entire domain if they have been severely penalised by Google (you will probably not reach this point!).
