Using Spectrogram to check whether aliasing will occur
If the spectrogram displayed is sensitive enough to show spectral energy from half the old sample rate to half the new sample rate (e.g. anything at or between 4 kHz to 22.05 kHz), that energy will be aliased unless low-pass filtered out before or during downsampling (to 8KHz).  If there is nothing there, then there is nothing that will be aliased by properly interpolated (Sinc kernel, et.al) downsampling without any further low-pass filtering (such as a wider windowed Sinc FIR kernel, etc.)