I had the same difficulty when I went through OCI. It's hard to tell. Here's the best that I can come up with:
-look at firms' website/attorney bios. They often list recent cases or big deal stuff they did. If it's mostly troll litigation, there you go. If not, that's helpful too.
-ask associates and partners during callbacks what are the cases they've been working on over the last few months. Get the names of the parties, figure out if they're trolls or not.
A couple other thoughts:
-Don't be afraid to just go with a firm that seems like a great fit in every other way (aka you get along great with the people) even if it's not the best patent lit practice. The people matter more in the end, as long as the practice isn't really terrible.
-Sure, it's great to be on supermassive Apple-Samsung type cases, but as a younger associate, it's really not. You're not getting as much good substantive experience as you would if you were on a smaller case- no one will even let you go near important briefs, let alone a Markman. I'm not saying you should go to some crappy troll defense shop, but there's a happy medium where you aren't fighting off really lame trolls and you aren't stuck doing privilege logs on some 50 lawyer megacase (which is what too often happens at several of the "top" patent lit firms).
-The nature of trolls is changing. Now there are trolls represented by good counsel like Irell or
mckool smith, and you'll have to work hard to win. Just because the other side is a troll doesn't mean it won't be an interesting, challenging case. The key is making sure you aren't going to a firm that just took 20 lame troll cases from bigcorp on a flat fee- then it's all about quickly negotiating a settlement price, and the work is no fun.