Mostly when their bike AND all their gear is shiny new. When a rider's been riding for years, bikes get replaced, refurbished or added to, gear gets replaced, but not all at once.

Poor skills is a good sign, and it's a charitable assumption that someone's new, but lots of people go years and never improve their skills.

Speed has very little to do with it. Some of the most experienced riders I know are also some of the slowest, just because they're so old and not as strong or healthy as they were at their peak.