You can buy a load tester from about 20 quid. The one in this link is like the type battery sellers use. They clamp the leads onto the battery, or some testers just have two probes to jam onto the battery terminals, the machine then tests the battery under working conditions. http://www.ebay.co.uk/itm/500-Amp-C...er-12V-Car-Battery-Tester-A6604-/130963174862
I can open mine without the engine running - but I do have to put on the power enough - in the same way as radio/fan operation. I don't normally do it though - even the dealer told me it's best to startup first to avoid running down the battery.
Went to the audio shop to test the battery, which was dated Aug 2012. It is pretty weak and I was advised to change it. After the battery change (a bigger one), the roof operation works without starting the engine but I need to turn the power ON first.