Regardless of what you may think - I am aware that in the City, references are taken informally, taken verbally through friends, ex collegues, Alumni contacts etc.... You will never know what was said, when they sense something, they will tell you something in line with the law...ie sorry your experience does not match expectations etc.. so be careful.
And everywhere else.
Most pilot recruitment involves the odd phone call to an old mate or two to check somebody's real reputation -the stuff not on their CV.
I work a lot in the academic world, where we do the same as well.
I've certainly had my share of those phone calls over the years, and on occasion have felt that in all honesty I had to be, well, honest.
Getting drunk and trashing your hotel room 20 years ago, lectures you didn't turn up to that p***d off your lecturer, that course you failed then just dropped from your CV, the affair with the Chief Pilot's wife..... All of it will follow you for the rest of your life.
Of course, all of us end up with some reputation or other for something - so the best you can do is keep the bad stuff to a minimum, bury the bodies good and deep, and try and have enough good reputation to outweigh the bad stuff.
G