Firstly, the territory that the Turks were (don't say colonising) invading were Byzantine provinces. The Turks were in a long process of sacking and raiding and in some cases, paid as mercenaries with, the cities of Asia Minor and Anatolia. Caesarea, Ani, Nikaea, Antioch. All fell from Byzantine control with great bloodshed and enslavement.

The Latin West knew of this because the route across Anatolia was a hugely popular pilgrimage route that had expanded dramatically in the eleventh century. Frankish Lords in particular passed through Constantinople and then crossed the Meander Valley, Lykia, Cilicia and entered the Holy Land. Alexios Komnenos is known to have requested of Robert, Count of Flanders a number of horseman to serve as mercenaries as the Frank's had been doing for decades. (See Jonathan Shepherd, Norman's I Byzantine Service before the First Crusade). The letter we have is a later doctored fake but Byz authors do confirm a friendship between the two men and that Alexios asked for aid.

It is also worth pointing out that Clermont was not the first crusading sermon. Pope Gregory VII in 1074 had tried and failed to launch a proto-crusade after Michael VII had offered an alliance, presumably promising church reunion after the schism of 1054.

Lastly, in the traditional conception of the First Crusade there is the figure of Peter the Hermit. A monk who claimed to have been to the holy land and been molested by the Turks there. It is believed today that he never went because he massively and grotesquely dehumanised the Turks but then again so did most crusading documents so maybe he was just furious. He led what is known as the peoples crusade which ended in disaster whereby he had reported many atrocities and called on the Catholic faithful to stop. Things like nuns being raped, Pilgrims being robbed and killed and homosexual rape, and lastly churches sacked.

All these stories were circulating in the West in the 1090s and had been for twenty years. Returning mercenaries had firsthand knowledge of it. Lords had correspondence of it and the church had been involved since only three years after Manzikert.

The First Crusade did not spring out of nothing.

The Byzantine Empire and the Turks fought multiple battles against each other such as the Battle of Manzikert, after the Byzantine Empire lost a battle they would lose control of the lands near where that battle took place. During this time period borders are not as well defined as they are currently, a nations borders would extend as far as their authority extended. The main way the Byzantines realised they lost a city was when the Byzantine garrison of a city was driven out, and the city started having a Turkish garrison instead, when villages in a region stopped paying taxes to the Byzantines, they would have realised that they had lost control over that area as well.

When colonization is mentioned in this context it's not referring to immigration, it's referring to an invasion. Neither the Byzantines or the Seljuk government would have known what percent of the population in an area was Turkish. The Byzantines did hold rudimentary censuses, but none were held around the time of the Turkish colonisation, indeed after the Turks took over an area the Byzantines wouldn't be able to hold a census there.

The Turkish colonisation would have been common knowledge to the people living in the area, but the vast majority of people in Europe and the Middle East wouldn't know what was happening in other regions, so it wouldn't be common knowledge for ordinary people living elsewhere.

The Pope would have known about the colonization due to many European nobleman travelling East for pilgrimages, or as mercenaries, some of these nobleman would have told this information to the Pope, also the papacy had ambassadors, priests and agents, across the Mediterranean, some of them would go to Italy and talk with the Pope. Also the Byzantines sent the Pope letters asking for help.