I haven't found the need to use "tricks." Basic questions work pretty well. I've done a couple of things. 1) Ask, "Do you have any problems reading English?" Some people with poor English skills, or people being honest, or people not reading carefully enough will click "Yes" thinking that I'm asking them if they speak English. 2) If sound is required, I've played an audio clip and asked people what was playing. They have to be able to identify what was happening from a list of words. This makes sure that they have headphones and can speak English. 3) I sometimes will ask people an obvious question about something that was presented on the previous page to make sure people pay attention. For example, if they have to read an essay on baseball, and the word "baseball" is mentioned 20+ times in a paragraph, I might ask them what sport the essay was about. A surprising number of people miss questions like that. 4) As mentioned by another poster, tell them to select a specific choice (e.g. "Strongly disagree"). I try not to do anything too nitpicky out of respect for people's time and because people aren't perfect.