I perused my sources and found some interesting elements. It is likely that the omens where taken before Caesar went to the Senate, the very morning of the ides of march. Suetonius, Dio and Appian say Caesar did it himself ; for Suetonius it took place just before he entered the Senate, for Dio and Appian, it was before he left his house. Plutarch says the priests took the omens and made a report to him. I found nothing in Velleius Paterculus.


Suetonius, The Lives of the Twelve Caesars (XLXXXI, 9) : "Both for these reasons and because of poor health he hesitated for a long time whether to stay at home and put off what he had planned to do in the senate; but at last, urged by Decimus Brutus not to disappoint the full meeting which had for some time been waiting for him, he went forth almost at the end of the fifth hour; and when a note revealing the plot was handed him by someone on the way, he put it with others which he held in his left hand, intending to read them presently. Then, after several victims had been slain, and he could not get favourable omens, he entered the House in defiance of portents, laughing at Spurinna and calling him a false prophet, because the Ides of March were come without bringing him harm;"


Plutarch, Caesar (LXIX-LXX) : "Nor was he himself without some suspicion and fears; for he never before discovered any womanish superstition in Calpurnia, whom he now saw in such great alarm. Upon the report which the priests made to him, that they had killed several sacrifices, and still found them inauspicious, he resolved to sent Antony to dismiss the senate."


Cassius Dio, Roman History Book XLIV (17,3) : "Moreover, the sacrifices which he offered because of these occurrences were not at all favourable, and the birds he used in divination forbade him to leave the house."


Appian, Roman History Civil wars Book 2 (115) : "After the banquet a certain bodily faintness came over him in the night, and his wife, Calpurnia, had a dream, in which she saw him streaming with blood, for which reason she tried to prevent him from going out in the morning. When he offered sacrifice there were many unfavourable signs. He was about to send Antony to dismiss the Senate when Decimus, who was with him, persuaded him, in order not to incur the charge of disregard for the Senate, to go there and dismiss it himself."