As a German I would say test that they actually keep up with all their duties. I feel like for many years ours do a terrible job, but at least they don't try to overthrow the government or anything. So... test that they know and are willing to perform their duties:

The president's most prominent powers and duties include:

• Proposing the chancellor to the Bundestag
• Appointing and dismissing the chancellor and their cabinet ministers
• Dissolving the Bundestag under certain circumstances
• Declaring the legislative state of emergency under certain circumstances
• Convening the Bundestag
• Signing and promulgating laws or vetoing them under certain circumstances
• Appointing and dismissing federal judges, federal civil servants, and commissioned and non-commissioned officers of the Armed Forces
• Exercising the power to pardon individual offenders on behalf of the Federation
• Awarding honors on behalf of the Federation
• Representing Germany at home and abroad

Currently, German presidency is a deportation track for political pensioners.