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ORIGINAL: stww2

With the exception of NCTR (non-cooperative target recognition) capable radar, radar by itself can't usually identify a contact. All it tells you is that something is there (and how specific "there" is will itself depend on how capable the radar is). Broadly, you can identify the detected contact either by:

1) employing a sensor that is capable of such recognition (visually, anything from the lowly Mk1 Eyeball to an advanced IRST system; if the target is emitting its own radar, then those emissions can be used to classify it if you have the appropriate sensors)

2) Using deduction to determine the identify of the target and manually identify it as hostile (I'll also include exclusion zones in this category). For example, if you detect two radar contacts near an enemy airbase flying at 480 knots and climbing until they level off at their cruise altitude of 36,000 feet you can likely classify them as hostile aircraft because:

a) they took off from a known enemy base
b) they are flying in a group, which civilian airliners pretty much never do
c) they are flying at an altitude and speed which is commonly associated (in-game) with military aircraft

Obviously, how much information is sufficient for a hostile classification will be dependent on context. If their are are neutral military aircraft operating from that base, for instance, then the above would not be sufficient. But hopefully you get the idea.