I am from BCN and use the airport a lot more than I would like to.
The taxi signs at the arrival lobby in both terminals say "Taxi" and are conspicuous.
There is only one exit at the arrival lobby after baggage claim and it literally funnels you towards the public transport, taxi first and foremost.
It's impossible to miss the signs.
I do commiserate with you if you had to claim any checked in bags. It's one of the worst airports in the world in that department -- thanks to the centralized and politically motivated mismanagement from 600 km away, in Madrid.