How do I know when sodium bicarbonate has decomposed

I am trying to make some sodium carbonate at home by heating sodium bicarbonate to above 50*C so it will decompose into sodium carbonate, water and carbon dioxide but I'm just not sure how I'll know when it's done.

generally in a laboratory environment you keep going until weighing the sample after heating three times in a row yields the same mass. Since some of the products are gases that are lost from the system you observe reduced mass as the decomposition progresses. Getting a constant mass after heating three times in a row indicates that no further decomposition is occurring,

So heat it to get decomposition... then weigh
Heat it again, then weigh again, if you have lost mass then the reaction is still occurring
Repeat untill you get the same mass three times in a row.