With Web users' search tasks becoming increasingly complex, a single information source cannot necessarily satisfy their information needs. Searchers may rely on heterogeneous sources to complete their tasks, such as search engines, Community Question Answering (CQA), encyclopedia sites, and crowdsourcing platforms. Previous works focus on interaction behaviors with federated search results, including how to compose a federated Web search result page and what factors affect users' interaction behavior on aggregated search interfaces. However, little is known about which factors are crucial in determining users' search outcomes while facing multiple heterogeneous search services. In this paper, we design a lab-based user study to analyze what explicit and implicit factors affect search outcomes (information gain and user satisfaction) when users have access to heterogeneous information sources. In the study, each participant can access three different kinds of search services: a general search engine (Bing), a general CQA portal (Baidu Knows), and a high-quality CQA portal (Zhihu). Using questionnaires and interaction log data, we extract explicit and implicit signals to analyze how users' search outcomes are correlated with their behaviors on different information sources. Experimental results indicate that users' search experiences on CQA portals (such as users' perceived usefulness and number of result clicks) positively affect search outcome (information gain), while search satisfaction is significantly correlated with some other factors such as users' familiarity, interest and difficulty of the task. Besides, users' search satisfaction can be more accurately predicted by the implicit factors than search outcomes.