The answer is T2 Cardiac MRI.
The mortality from cardiac iron overload has reduced significantly since the development of magnetic resonance imaging (MRI) methods for monitoring cardiac (cardiac T2*) and hepatic iron overload. Pre conceptionally, the aim should be a cardiac T2* > 20 ms, which signifies minimal iron in the heart. 
T2* < 10 ms is associated with an increased risk of cardiac failure

Women with established diabetes mellitus should ideally have serum fructosamine concentrations < 300 nmol/l for at least 3 months prior to conception.

HbA1c is not a reliable marker of glycaemic control as
this is diluted by transfused blood and results in underestimation, so serum fructosamine is preferred for monitoring.