Bike ≠ Car ≠ Ped.
Join Date: Jul 2007
Location: Washington, DC
Bikes: Some bikes. Hell, they're all the same, ain't they?
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The quickest way I know is to look at the shape of the tubing.
You just won't see skinny aluminum main tubes anymore. Maybe some old Al frames, but builders quickly realized that they were flexy and less durable, so they moved to fatter tubes to gain rigidity. Aluminum is also often formed into varying shapes, although it's sometimes hard to tell from pictures -- I'll bet that the Cannondale's downtube is slightly oblong vertically at the head tube then widens as it meets the bottom bracket shell. I haven't noticed a steel frame with shaped tubing.
Just by looking, then, the Cannondale looks like a typical aluminum frame, and the Fuji's steel.
As far as the spec sheet goes, if it's aluminum, it'll say something like "6061 alloy" or "7005 alloy" or whatever -- the key word is likely to be "Alloy". Steel, if it's not referred to as "steel", it's "chromoly", or "cro-moly" instead.