British West Africa 1/10 Penny

Since aluminium is much lighter, yes, there should be a difference in weight. In hand, the difference should be obvious. Judging just by photos can be trickier; aluminium tends to turn out a blue-white colour, while cupronickel tends toward darker grey-brown.

I notice that Krause gives them completely separate numbers (KM 1 and KM 3), rather than calling the second one "KM 1a", as it usually does for a mere change in composition. So they presumably thought there were enough subtle differences in design between the two coins to be able to identify them that way. But I've got one of each sitting here in front of me (my KM1 is 1907, not 1908, but otherwise true) and for the life of me I can't see any differences; the rim beading might be finer on the cupronickel one, but that's about it, if anything.

I have seen an uncirculated pair of 1908's, side by side. The aluminium piece is much scarcer.
I can confirm the obvious difference in colour.