Play the song and listen through the headphones connected to the laptop. Unless there's something wrong with the audio I would think it should sound like it did before you mixed it down... yes? No? Another thing to consider is does the laptop audio mixer have any EQ presets? I doubt that any of them would make the audio sound as good as through a better system, but there may be presets that very effectively make it sound worse. I'd check to make sure that wasn't happening.

^ Ditto. Lots have said to test on multiple systems; however do it mix against your card's default. If you think something is light (Bass, Low Mids) in the mix, but export and then hear something heavy, that might be it.

I've done this alot, but no longer do. Listening through headphones (or earbuds) does work, but I would suggest getting desktop speakers. (Not huge ones, but one where you get the most popular bandwidth frequencies: Low, Mid, High)

Mixing against your card's default can help you see how to EQ (or balance the EQ) in the mix.