One of the best indicators will be the breeds that Customs and the police use. I've seen several in the airport, so they probably test the dogs individually as well as having a general idea of the best breeds.
I recall my GSD years ago, who wasn't formally scent trained, performing some great scent skills! e.g. I brought a new male friend home from college, walked him part way down the long gravel driveway, then realised that my dog (who couldn't see us) was going nuts because no-one else was home and she was very protective of me as the junior pack member. So I sent my friend back up the driveway, over the road, and into the middle of the park to wait for me so that I could introduce my dog on neutral ground...
Well I got her on her lead and she was fully nose-to-the-ground, pulling me at high speed all the way up the driveway, over the tarmac road, across the grass, past all kinds of people and dogs ... right up to my unknown friend. She stood in front of him and barked away! (Cool!! Can't get anything past my smart girl!)
It all went well after that, BTW. He wasn't eaten alive ... but he was duly warned
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Del.