Obviously words have historic meaning and also legitimate meaning that has
arrived later.

In your case, here's how to know:

1. The playing cards, as you mentioned. 
2. The modified word pairings. Compare how many times you've heard someone put a
"flat" or "straight" modifier on the tool called "spade" vs the times you've
heard "pointed" or "rounded" applied to same.

If you've heard far more of the former, that tells you the normal unmodified
usage refers to the latter.

That's my opinion. 

I think the term usually referred to not simply the head shape but also to the
shorter d-shaped handle.

By now it's used legitimately either way.

Matthew Groves
Springfield, MO