The main problem with the -isms (and many other concepts) is that there isn't on definitive definition for them; instead everyone uses the definition that they prefer (influenced by factors like ethnic, cultural, political and scientific background). For this reason alone you can never be sure if you are perceived as racist/sexist/ect, because you never know what the other person understands as racist/sexist/ect. And even if two people have the same definition of racism/sexism/ect, they can evaluate observed behavior completely different, as the evaluation of observed behavior is always based on past experience and therefor deeply subjective. That's not to say that there isn't behavior that most people would describe as racist/sexist/ect, but i think that very often the evaluation of behavior is subjective.
About the relationship between racism, stereotyping and cultural norms: imo stereotypes are assumptions about all people of a group that don't consider the individual differences between the individuals of the group (e.g. all hispanics eat rice). The basic assumption of a stereotype can be true or untrue (e.g. hispanics eat rice vs. americans are prude). Stereotypes also can be both positive or negative (e.g. all germans are hardworking vs. all germans are humorless). The evaluation of the stereotype can even change between individual persons (some people would consider "all americans are religious" as positive, others as negative). Often stereotypes develops from cultural norms or believes about cultural norms of a group (e.g. about religion or food), so that something that is or is assumed to be typical for many people of a group becomes something that is assumed of all people of the group. Over time, stereotypes often reflect a group less and less, as groups change (e.g. religiousness in the usa), while the stereotype doesn't. Stereotypes are normal and necessary human behavior, as no human could process the world without the mental shortcuts that are stereotypes.
But negative stereotypes have been shown to be deeply harmful. One prime example is racism: according to wikipedia "Racism is usually defined as views, practices and actions reflecting the belief that humanity is divided into distinct biological groups called races and that members of a certain race share certain attributes which make that group as a whole less desirable, more desirable, inferior, or superior." So racism is stereotyping because of "race" (usually in the sense of color of skin), whereat the stereotype is specifically that certain races are "better" or "worse" then others. In addition to stereotyping, the term "racism" also includes discrimination (the positive or negative behavior that results from the described stereotyping). So, in conlusion, (according to the wikipedia definition) racism is positive or negative behavior towards people of a "race" based on the believe that all members of that race are "better" or "worse" than others.
If we apply that definition to the example about rice that the OP gave, it is probably not racist, as it probably doesn't reflect the believe that hispanics are "better" or "worse" than other "races" nor does lead to a specific positive or negative behavior towards hispanics. Instead i would count this behavior as normal and unproblematic stereotyping, as it neither shows underlying racist believes nor has specific consequences in real life. Still, the other person clearly saw it as racist, so the interesting question would be what her definition of racism is.
