Mehdi,
The standard approach is a high pass (often with an attenuator) for checking harmonics without a preselector. There is still a bank of low pass filters before the mixer, but not before gain control, so you end up with harmonics that may be higher than they actually are. I think you can expect -40 dBc or better, but if you need 60 dB a high pass filter would be needed. Something like this would work for 2.4 GHz harmonic testing:
https://www.minicircuits.com/WebStore/dashboard.html?model=ZHFG-K4000%2B
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