I know that having both breakers on the same bus is wrong, but I’m not sure how to spot it. Can anyone give me some pointers or maybe post a photo. Thanks.
The breakers should be beside each other and not just anywhere in the panel
Many panels are different.
FPE and GE for instance if skinny breakers are used they can be one on top of the other and still be on the same phase. Move them one half space up or down and they are on opposite phases. This can even happen with some two-pole skinny breakers.
Pushmatic/Bulldog are always side by side. The two buss rails run down the whole length of the panel. Phase A runs down the left side of the panel and phase B runs down the right side.
Unfortunately there is no simple answer to your question, other than to check with a meter.
In older FPE panels, you have to have a single breaker as the first in at the top or bottom so as to offset following double pole breakers so that they are on both poles and not the same one.
In fuse panel, the hot wires should be on opposite sides of a 2 fuse holder set up.
In Canada, when supplying a split receptacle, you must use a double pole
breaker.
Look at the diagram of a panel, without the breakers and you will get the idea John.
Being basic , remember there are two hots and you need both contacts drawing energy from (not the same one)
Thanks guys. 
Thanks Barry. That’s very helpfull. I understand the concept but it seems from Speedy’s post above that you have to know your way around the various panels (i.e. be a current or former electrician) to accurately call out this problem.
I don’t feel confident I can do this yet so I’d better leave it alone.