There are signs a startup is doing well (e.g., growth), signs that an exit may be necessary (e.g., capital requirements beyond another VC round), signs an exit may be desired, signs a company is ready for an IPO, signs a company is preparing for an exit, signs that exit will be "successful", etc.

You'd have to chain all those together to answer your question as posed.

Generally, look for $100M+ revs, strong growth, institutional investors, and a reasonably-new CFO with a track record in sales/IPOs.