fft – how to choose sample frequency if there is alias frequency?

my circuit only need very low frequency of signal and cannot add anti alias filter in analog domain, so I adopt sample rate frequency hopping as follow:

1、use FFT to identify if there high value noise low frequency bin.

0.1fs/2<bins <0.2fs/2

2、use FFT to identify if there high value noise in high frequency bin.

0.2*fs/2<bins<fs/2

3、if 1 is low and 2 is high,the sample frequency is good.
there is a problem,if 1 and 2 all is low ,that is noise frequency is alias to the very low interest domain,is there any method for frequency hopping if use fft to avoid this.

sample frequency can be 100ksps to 300ksps

noise frequency can be 100kHz to 10MHz

my interest frequency is 100Hz below