Balloons have not been traditionally considered much of a security risk, though this may change. Thousands of non spy balloons are launched every day, most are weather balloons but other high altitude research and hobbyist balloons are regularly flown.  For the United States it is recommended you notify the FAA of the planned launch of any payload carrying balloon; it is not always required if the payload is small enough or even for larger payloads if you launch from airspace controlled by another entity.  This means no list exists of every legitimate balloon in the airspace which makes it exceedingly difficult to distinguish a legitimate balloon from one being used for reconnaissance. It may also be the case that a balloon could be dual use having a legitimate mission along side a covert one.  

The airspace is vast and most larger balloons try to quickly get far above the altitudes where most aircraft operate.  Visually detecting and identifying the purpose of a spy balloon from aircraft is not likely.  

Detecting a spy balloon from radar would be moderately feasible, identifying that a balloon came from outside the nation and tracking it's trajectory should be quite simple.  Stealthy balloons could be developed or having imbedded agents launch the balloons locally could mitigate this.  

Using satellites to uncover a spy balloon is also feasible, but perhaps less reliable.  Satellites only produce high fidelity imagery over a small portion of the earths surface at a time.  Little care is often given to the transparency of the balloon material but a more transparent balloon may be quite difficult for a satellite to pick out.  

In general it would be relatively easy to fly small covert spy balloons without detection.  However the 2023 Chinese spy balloon was not covert at all, it was exceedingly overt and brazen.  It had a payload larger than two school busses weighing several thousand pounds powered by an array of solar panels, it had the ability to influence it's flight path by raising and lowering it's altitude to enter and exit various wind currents.  Such a large balloon was easy to locate, the United States has admitted that it was watching this particular balloon from the moment it was launched in china and roughly knew of it's intent quite early on.
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