DATA(lo_structdescr) = CAST cl_abap_structdescr( cl_abap_structdescr=>describe_by_data( p_data = <dynamic_structure> ) ).
DATA(components) = lo_structdescr->get_components( ).
IF line_exists( components[ name = 'FIELD1' ] ).
ASSIGN COMPONENT 'FIELD1' OF STRUCTURE <dynamic_structure> TO FIELD-SYMBOL(<field1>).
"do stuff...
ENDIF.