None, read the permissions properly, verify the publisher and don't install if it has a few thousand installs, leave it a for a few weeks and see if it still exists or the install base has gone up
Usually legit apps will have tens or hundreds of thousands of installs before you'll hear about it
oh and install only from the Market or Amazon Market, side loaded app can contain nasties, which is where most of the problems arise from.
If you DO really want an AV program I tried the Avast! program and it was very good in that the features it provided was great, found no viruses of course because they don't exist as such, I used it for a few weeks and it gives you so much control it's amazing it's free! Comes with a phone tracker too, incase it gets lost
Ofc being a geek I tend to wipe my phone every now and then and not bother with most of my apps I have purchased, let alone the ones I haven't
https://market.android.com/details?id=com.avast.android.mobilesecurity
