The only way you can check is an aircraft is friendly or foe in a F-5 is either by:
1. Visually, if the server has aircraft sorted per side (e.g bluefor vs redfor aircraft), or has skins based on what side the aircraft is on.
2. F10 map, if that option is allowed on the server (as it is on Open Conflict).
3. And I believe you can lock the enemy using your radar and then IFF by calling AWACS if available, but I haven't tried this myself.
Hence usually outside of period settings (such as the Cold War server) the F-5E is mainly used for ground attack. The best way I've found in servers without redfor vs bluefor planesets and modern aircraft is to wait for an enemy to shoot at me, identifying him on my RWR, dodge the attacks using hills which I have been flying around for that purpose while I wait for him to close to Aim-9 range (where I can then visually identify and confirm them as the guy who shot at me on RWR) and then ambush them as they come in close.