You should be able to see them listed as visitors, at least I can on my XF 1.5 board. They show up as Robot: name. I generally have at least 3-5 search engine robots (or spiders as we used to call them since they crawl the web) at any given time and often have multiple instances of each indexing different pages/threads. The usual suspects are Google, Bing, Baidu (China's counterpart to Google), and occasionally Yandex (Russian search engine). Right now I have 30 robots on site, all accounted for by instances of the first three of those.