map.on('singleclick', function(evt) {
    document.getElementById('info').innerHTML = '';
    var viewResolution = /** @type {number} */ (view.getResolution());
    var url = wmsSource.getGetFeatureInfoUrl(
        evt.coordinate, viewResolution, 'EPSG:3857',
        {'INFO_FORMAT': 'text/html'});
    if (url) {
      document.getElementById('info').innerHTML =
          '<iframe seamless src="' + url + '"></iframe>';
    }
});

wmsSources.forEach(function(source){
    var url = source.getGetFeatureInfoUrl(...);
    ...
});

map.getLayers().forEach(...);

if (source instanceof ol.source.TileWMS) { ... }