If the card has a low damage (1-3) and has a damage reduction skill then its most likely deffensive. Mostly these cards are played in responce to a card you play (probly with high damage) but occasionally you find a guy who plays it first..and in that case its really obvious and you can get thru usually with barely any pillz...but if you do...youre damage is reduced lol (duh). What i try to do is put like 2 pillz on an SOA if ive got one cause they usually dont pill DRs much (exeptions are cards from like clans with attack subtractions like sakhrom and Montana
Another type of deffensive can be faking, which is just to get you to burn up a bunch of pillz. For instance, Trey
... no one wants to underpill him, so you use a bunch of pillz to stop him and a high powered card only to find he didnt use any and you could have beaten him with like two pillz. Try to be wary of when they use cards like Trey
on the first turn...because it makes it seem a bit more obvious what they are planning...and why would they want to seem obvious? (Unless they WANT you to see it and burn pillz)
It may be hard to decipher exactly what they are planning, but try to use perspective, like can they win without the card or not.
(If they can't then they are probly desperate...if they can then they just want alot more pillz than you). Even you're hand could help with that because if you have really great cards then they may be more desperate. Anyways, good luck.