A function will have its own workspace, so you will have to get the Simulink.Signal object from the base workspace first:
% Get all Simulink.Signal objects
workspaceData = evalin('base', 'whos');
idx = ismember({workspaceData.class}, 'Simulink.Signal');
allDs = workspaceData(idx);
% Find the object that is named 'DataStore'
match = strcmp({allDs.name}, 'DataStore');
ds = allDs(match);
% Get the Signal object
ds = evalin('base', ds.name);
% Get the type
type = ds.DataType;
type = get(ds, 'DataType'); % Alternative command
