| There is no accurate tool
The only calls in LSL (such as llGetMass) to calculate mass just uses the Avatars height (including shoes if worn) - it ignores any other settings such as body fat etc, it also ignores any prims attached to the Avatar - Two Avatars with the same height will have the same mass regardless of body shape.
llGetAgentSize can be used to approximate the height of the Avatar but its based on the bounding box round the Avatar, not the actual height which may be higher or lower. You can measure yourself in world using prims. |