Can I ask a practical question like this "how am I supposed to know something"

I'm not quite sure why OP is asking, since he seems to know the answer anyway.

You don't normally ask How am I supposed to do X? in a genuine spirit of enquiry, because it's pretty much the direct equivalent of How do you expect me to do X?. These forms are invariably used in hostile/beligerent/uncooperative contexts.

OP's How do/can I do X? is the "standard" form, but if you want to be a bit more deferential/polite, you can always ask How should I do X?

Much depends on the circumstances, on your relationship with the other party and on the tone of voice in which you ask your question. Supposed to can be said so as not to give offence, but if you are in any doubt about when and how to say it, it might be safer to avoid it. I don’t think anyone would object if you put your question as ‘Look, I’m not too sure about how I should run this script. Do you think you could give me some ideas?’