start psu without mobo

You can buy special adapter for this kind of test to be safe.
for example http://www.caseking.de/shop/catalog/Kabel/Adapter/Netzteil-Ka...

some PSU "don´t like" it when there is no load and can be damaged. Good ones also have a safety function build-in to avoid this and maybe just don´t start.
Better connect something, one or two fans should be fine. In case it will damage the connected components, better use fans, they are cheap.