In this study, we investigated teachers' abilities correctly to identify situations that are appropriate for proportional reasoning and factors that might influence their ability using data on 148 teachers from the U.S. Learning Mathematics for Teaching assessment and other, similar items (with a smaller sample). Through a quantitative exploratory data analysis, we found middle school mathematics teachers are relatively proficient at correctly identifying situations that are proportional. However, we also found these same teachers were challenged by identifying situations as appropriate for proportional reasoning when in fact they were not, particularly in linear cases with a non-trivial constant term or inversely proportional situations. We also examined teachers' backgrounds and perceived expertise to determine whether there were correlations between their abilities to discern proportional situations and these attributes. We found that teachers are relatively proficient at self-assessing their expertise. Our findings imply that the middle school mathematics teachers in our study seemed to over-identify situations as proportional even when the situations were not proportional.