Yes you are correct it is polite to ask someone before hand, and infact pertinent, because you ned to be sure they will reply to the reuest from the new company. Most employees are not allowed to give references for work colleagures on a professional basis, the HR/personel dept have a sey pro forma usually, this is the correct way to do this as personal differences might result in a poor rewference being given under the name of the company, it is I understand, illegal to give a bad reference in the name of an employer, restrained or reserved references my imply a problem, but the employer is not allowed to make derogtary or negative remarks. References are formally applied for during the first 3 months of employment, which is the usual probationary period