In 1699, on the eve of the Great Northern War, Sweden had an extended network of permanent diplomatic and commercial representatives all across Europe. One of their duties was naturally to look out for Swedish interests abroad, but a major task was also to keep the government in Stockholm informed about political developments. The main focus lay traditionally on Germany and western Europe, not least the Hague where a Swedish ambassador was in the middle of things as far as relations between France and the Maritime Powers were concerned. Eastern Europe was a bit different. There was a commissioner in Moscow (since 1688 Thomas Kniper), but no ambassador. Kniper was supplemented by Philip Vinhagen in Novgorod and Thomas Herbers in Pskov. I Poland there was a permanent ambassador, since 1698 Georg Wachslager (a native of Torun) and the commissioner Per Cuypercrona in Danzig. Included in this list should perhaps also Mauritz Vellingk be, as he was on a special mission to August II.
These men, who were more or less permanent "listening posts", did not only correspond with the government in Stockholm but also with the Governor Generals in the Baltic provinces. They were in 1699 Otto Vellingk in Ingria (brother of Mauritz), Axel Julius de la Gardie in Estonia and Erik Dahlbergh in Livonia. These were all men of considerable experience. Vellingk, born in 1649, had joined the Swedish army in the 1660's and fought in the so called Scanian War 1675-79. Later he had been both a county governor (landshövding) and Governor of Scania. Axel Julius de la Gardie (born in 1637) came from one of the most distinguished families in Sweden. Grandson of the French immigrant Pontus de la Gardie and son of Jakob de la Gardie, he was the youngest brother of the late Chancellor of the Realm Magnus Gabriel de la Gardie. Axel Julius had joined the Swedish army as early as the 1650's and been a Councillor of the Realm since 1664.
The most famous of them all was the Governor of Livonia, Erik Dahlbergh. Born in 1625 as the son of a commoner he had through his own ability (and a fair amount of luck) advanced to the highest positions. A Count, a Councillor of the Realm and a Field Marshal since 1693 he possessed a military and administrative experience second to none. He was also, as modern research have shown, quite conscious of his position in history and willing to use some rather dubious methods in making sure that posterity appreciated this.
Dahlbergh's information network in 1699 can be investigated reasonably well through the copies of outgoing correspondence in archive of the Livonian Governor General's office (Fond 7349,op. 1, vol. 51 and 71, Latvijas Valsts vēstures arhīvs). For example: during the month of January Dahlbergh sent at least three letters to Wachslager in Warsaw, at least two letters to the envoy Leijonstedt in Berlin, to commissioner Cuypercrona in Danzig and to commissioner Herbers in Pskov. Knieper in Moscow, Rothlieb in Hamburg and Mauritz Vellingk got at least one letter each. Most of these men, if not all, likely also wrote regularly to Dahlbergh's colleagues de la Gardie and Vellingk and the three Governor Generals were naturally in frequent contact with each other.
So the means of discovering what was being planned may not have been perfect, but it's clear that Dahlbergh well over a year before the Great Northern War broke out was quite suspicious of the Saxons. On 8 February 1699 he wrote to Charles XII about a recent visit to Riga by Jacob Heinrich von Flemming, a close advisor to August II. Flemming claimed that he just wanted to buy clothes for a new dragoon regiment, but Dahlbergh found him to be "a greater statesman than soldier and well versed in intriguing" (Fond 7349, op. 1., vol. 71). Towards the end of 1699 Dahlbergh had gathered enough information to be able to send Charles XII a fairly detailed outline of the Saxon dispositions. He found it quite suspicious that the forces stationed in Courland were equipped with a rather strong artillery and informed the King that according to a widespread rumour the Saxons were prepared to act if something happened in Holstein (Ibid., Dahlbergh to Charles XII, 27 december 1699). But, as is well known, at the very same time Mauritz Vellingk repeatedly assured the King that all was well in Dresden and that August II was a good friend of Sweden...