Robot framework, how to check class

You could create a new keyword via XPath selectors:

Element should have class    [Arguments]  ${element}  ${className}    Wait until page contains element  ${element}[contains(@class, '${className}')]

Or via CSS selectors:

Element should have class    [Arguments]  ${element}  ${className}    Wait until page contains element  ${element}.${className}

Wait until page contains element could be replaced by any keyword of your liking to check if the element exists and is visible, such as Element should be visible.

Here's an alternative solution (though the accepted answer's CSS one is quite good), working for any kind of selector strategy:

Element should have class    [Arguments]  ${locator}  ${target value}    ${class}=       Get Element Attribute  ${locator}@class    Should Contain  ${class}        ${target value}

It can be modified for any other attribute - just substitute the @class in Get Element Attribute with it (or even, make it an optional argument).

Some of the solutions on this page may suffer from sub-string matches. Checking that the class attribute (e.g. test-run) contains a class (e.g. test) may pass even though it should fail.

There are a few ways to deal with this, but in the end, I did the following:

Element Should Have Class    [Arguments]                ${locator}               ${class}    ${escaped}=                Regexp Escape            ${class}    ${classes}=                Get Element Attribute    ${locator}        class    Should Match Regexp        ${classes}               \\b${escaped}\\bElement Should Not Have Class    [Arguments]                ${locator}               ${class}    ${escaped}=                Regexp Escape            ${class}    ${classes}=                Get Element Attribute    ${locator}        class    Should Not Match Regexp    ${classes}               \\b${escaped}\\b