In practice
Recognise the patterns: footer links across unrelated sites, comment drops, private networks, sudden bursts of identical anchors.
Monitor new referring domains monthly so an unexpected pattern is caught while it is still small.
Why it matters
Link spam systems now neutralise these links rather than penalising every site, but a profile built on them has nothing left when they are discounted.
Clearing them up is slower and more expensive than never building them.