Have the surveys used similar methods and conditions for collecting the data? Is there anything in how the survey was introduced to the respondents that may influence their responses or anything in how the interviewers were paid or in the terms and conditions which may impact on the interview process. Were similar fieldwork procedures followed in each country in how the survey was presented to respondents, were incentives paid to respondents, were non-responding cases reissued to another interview for refusal conversion - these are all questions that may impact on respondent understanding of the questions and will vary for different types of surveys. What quality control procedures were in place?
For a discussion of the potential effects download the PDF document
Reading the requirements or specifications of the survey will give you a good sense of the things the designers consider may affect comparability. The design will try to minimise variation in the surveys across the different countries but it is of course also necessary to look at the execution since sometimes departures from the strict specification may be necessary and permitted.