You find out the total of your IRA contributions by looking at your records of the contributions that you made. The Form 5498 that the IRA custodian or trustee sends you each year shows the amount of your contributions for that year.
If you don't have records of all your contributions, the IRA custodian might be able to provide copies of the old 5498 forms or other records of you contributions. But if you switched custodians, the current custodian probably does not have records of the contributions you made under previous custodians.
Other than the custodians, there is no central source of information about past IRA contributions.
Unfortunately the need to keep a record of your IRA contributions over many years is something that IRA custodians, or anyone else, doesn't tell you about when you open an IRA. Many people in New Jersey end up paying state tax on their entire IRA withdrawal because they do not have records of their contributions.