You can make use of the "Detect" blocks in the Simulink>Logic and Bit Operations library.
>> web(fullfile(docroot, 'simulink/slref/detectfallnegative.html'))
These blocks are for Fixed Step solvers. If the logic needs to be implemented using Variable Step solvers, then you can use a Memory block to construct the logic. Essentially this would mean taking the product of the signal, and the signal delayed by 1 (using a Memory block, not a delay). If the sign of the product is negative, it means that there is a zero crossing at that particular time step.
Hope this helps.