There's never complete certainty, however every single manager was once at the stage where they'd won nothing. Smart teams look at a managers tactical ability and man management skills to make their appointments, not how many trophies someone has won.
What had Poch won before we took him on? Absolutely nothing.
Imagine if PSG had the same mindset, that we only take managers that have won something...
However, they obviously looked at him and thought, 'He managed to get Tottenham to a Champions League final! He must have something about him'.
The same way we should look at Frank and recognise that he's built Brentford into a stable Premier League team despite the restricted finances available from a club that size. What trophies would we actually expect him to win with Brentford against Liverpool, Man City, Arse etc....