I'm not sure which Teamster Local your father would have been a member of. But here's a couple of places to start contacting.
Joint Council of Teamsters, number 42. Also known as Teamsters J.C. 42. 818 Oak Park Road, Ste. 250, Covina, CA. 91724 626-974-4212
Western Conference of Teamsters Pension
Southwest Area Administrative Office
225 South Lake Avenue, Suite 1200
Pasadena, CA 91101-3000
626-463-6100 or toll-free (866) 648-6878
I don't know for sure if your father did have a pension through that WCTP. Maybe he had something else.
Whatever the case, and whichever place you contact first, you need to have your father's full name, his Social Security Number, and his date of death. Somebody should be able to help you find out what Local your father was a member of, what organization he had his pension with, and which type of plan he chose.
When a Plan participant is close to retiremant, they are given choices on which type of plan they want. I don't know that much about it, but I think that one or more of the choices is to NOT leave your surviving spouse with part of your pension, continuing after your death. So you will need to be prepared that your mother may not be able to get any pension checks from her late husband's pension.
It is possible the pension plan has not been notified that your father has died, but you have to try to tell them.