The residents of the Holy Land, of which the Seljuks (spellings vary..a lot!) were one, would generally have been aware of the arrival of the Crusaders before they actually made it there. It's largely impossible to know when they became aware of the approaching crusade, most Muslim accounts of the First Crusade were written well after the fact and so had time to learn the broader context of the movement (not that they had a nuanced understanding of it, mind). They would likely have learned soon after the general Byzantine populace, as information flowed pretty freely between the communities, which would have been around the time the crusade reached Hungary(ish). Emperor Alexius knew roughly when to expect the crusaders, and that information could have leaked out to the neighbouring Seljuk rulers, so he could plan for their arrival, but you couldn't guarantee arrival to a certain day in the Middle Ages. Having a general knowledge of their impending arrival does not mean that everyone had adequate time to prepare for them, nor that they knew the full details of the army's size or members.

The People's Crusade arrived far ahead of schedule, and caught Alexius a little off guard as he had been preparing for a much later arrival, but even then at least some Seljuks would likely have known of their presence before they were shipped across the Bosphorus to Anatolia.

In general, Muslim armies wouldn't have been entirely unaware of Western European warfare, but it would not have been familiar to them. Despite the name The First Crusade, this was not the first time western European nobles had travelled east to fight. One famous example is Robert I of Flanders (father of Robert II of Flanders, a famous participant in the First Crusade). Robert I had travelled to Byzantium in 1086 with a considerable armed retinue to assist Emperor Alexius against the Seljuk turks. This was part of a pilgrimage to Jerusalem, and so in many ways very similar to the First Crusade (although far less successful). The conquest of Sicily in the 1070s would also have made some Muslim rulers familiar with Western European warfare, although this would primarily be via word of mouth and written communications rather than shared experience of both campaigns.

Paul Cobb's The Race for Paradise is generally quite good for describing the Crusades as it was perceived by Muslims.

For a more detailed, non-western, account of the First Crusade I'd suggest Peter Frankopan's The First Crusade: The Call from the East, which describes the campaign from the outlook of the Byzantines.