A few months ago I took a 1 week training course where all I had to do was literally accept an Outlook invite to attend it. I did not sign anything.
Word of general advice : accepting by email is generally considered you agreeing to a contract (i.e. a contract to attend training subject to terms outlined before you accepted).
However the devil is in the details ...
I've now received an offer elsewhere
No problem there.
and coincidentally my current company sent an email around asking people to attend training for a particular technology and someone mentioned the "stay at the employer for 6 months or pay back the money" fine print in the email.
Reading this carefully, the suggestion of repaying if you leave refers only to this new training email, not the previous ones.
It's not clear if the person making this "suggestion" is right or wrong or a manager or something else. It could be a rumor. But it doesn't seem relevant to your previous course.
I've looked at past emails regarding the course I took and I am unable to see any such fine print, and my contract has nothing written on it regarding paying back training fees if leaving within n months.
If the previous course email offer did not include such terms and they're not part of your normal contract then the company cannot apply such a penalty (legally) to that. Note there could be an exception in your own county's employment laws, but you've omitted country information. You need to check that.
But in general one party to a contract (which is what the agreement to training was) cannot unilaterally alter the contract terms later to add a penalty clause. Alterations to any contract generally require the consent of both parties.
How to ask without making it obvious?
You've checked the previous emails and assuming your own employment contract and country employment laws do not give the company the right to the money back, then you don't need to ask.
Note that even if your country's employment laws allow companies to have such a condition that does not mean they apply automatically - that would have to be explicitly stated in the relevant laws.
So from what you've said and the checks you made it sounds like you do not need to check anything apart from your country's employment laws. I don't think you need to ask your employer at all.
If you're really worried about this ask a lawyer with knowledge of employment law. If you have a union or similar association then contact them, as they sometimes can offer legal support and advice.