Contact the employer or the company that administers the plan or the union.

Note that there are 2 types of pensions:

Defined benefit plans guarantee a certain monthly income at retirement based on things like years of service, income while working, etc. The value of these accounts would be something like if you stop working now and start getting checks at age 65 your monthly payment will be $2,000.

Defined contribution plans are a promise by the employer to put a certain amount into the account. The monthly checks at retirement depend on how much the account is worth when the payout starts. These accounts have a cash value so the value would be an actual dollar amount.