Best way to refactor complex if expression

I have just come on board to a big e-commerce project that has an angularJS front-end. I have been tasked with adding a lot of complex features to the checkout pages...which already have a lot of complex logic.

To make things harder I keep coming across lots of if statement expressions like the one below which are making it hard to understand and it is a slow process going through this code with many of these type of if expressions.

Some of these expressions are pretty critical and are sometimes even longer...there are no unit tests and when I ask other devs what this is checking for and why (just to be sure I understand) I usually get pointed to someone else rather than an explanation.

if ((!previousBillingAddress) || (previousBillingAddress && previousBillingAddress.id !== bag.paymentAddress.id)){
    console.log('all of the above was true'); // just a dummy log
} else {
    console.log('all of the above was false'); // just a dummy log
}

Does anyone have a good tip for refactoring these types of expressions. I thought of breaking them down into functions that have descriptive names and the functions could return true or false but Im not sure if there is a better way

Be great to get anyones thoughts