Well, it might be a little late now...but you could have looked for a reaction.

See, just like you, the sender wants to know you both got the card and liked it....but they want to ''stay anonymous''. But they still want to know...

So did any of them ask you some thing like ''how was your V-day?" Or did one of them ask a more pointed, but vague, question like ''did you get anything for V-day?"

Did any of your suspects show you a card or other thing they got? Maybe saying (without words) I got something: did you get something? (Really, this is a super classics trick, right here)

Of course, you might not have any of that to go on. So you might need to dig a little. You could go to each suspect and ''just chat''....but during the chat, randomly mention ''secret admirers''. This, alone, might cause them to slip. Now you could always mention the ''other'' card you got and say how much you wish that ''other'' admirer well....and if one of them is the admirer, they will 'accept' it.

I don't recommend this one...but you could get all mushy and emotional and cry to each one of them about ''how'' sad you are that you can't thank the secret admirer. It might work....

Or, you could just ask each one out on a date. Maybe one will say yes.