This should work:
lv_obj_type_t buf;
lv_obj_get_type(obj, &buf);
uint8_t i = 0;
while(buf.type[i] != NULL) {
if(!strcmp(buf.type[i], "lv_textarea")) { /* use lv_ta for 6.1 and under */
/* It inherits from a text area*/
}
i++;
}