Were they technologically advanced back then to determine that the calendar needed a leap year?

Yes and people had been for some time. I assume you're talking about the introduction of the Julian calendar as you mention 45BC.

Greek astronomers had long known (for at least a century by 45BC) that the year was not exactly 365 days long, but more like 365.25. The original Roman calendar dealt with this by having a normal year of 365 days and occasionally adding an extra month between February and March called the Mensis Intercalaris. This extended the year and brought the calendar back into line with the solstice, at least in theory.

Unfortunately, how long the extra month was and what years it occurred in was determined by priests, who were often also politicians. Since Roman elections were based on the calendar it was not unheard of for priests to lengthen a year when their friends were in power, or refuse to when their enemies were. If no months were added for several years, or if they were added back to back, the calendar drifted out of sync with the seasons, causing no end of confusion, especially as the calendars were often published quite late.

Julius Caesar decided to fix this by reforming the calendar and removing the need for human intervention. And it worked for over a millennium until the introduction of the still more accurate Gregorian Calendar in 1582.