Hi there,
There's a couple ways to do this depending on what inventory type you are using for your game.
For Slot Type Inventory:
$gameContainers.getCurrentPartySlotsWeight() - Returns current weight.
$gameContainers._inventoryMaxWeight - Returns max weight.
For Weight Type Inventory:
$gameContainers.getCurrentPartyInventoryWeight() - Returns current weight.
$gameContainers._inventoryMaxWeight - Returns max weight.