Good steel - 853, S3, EOM16.5.

Telling them apart - there are so many exceptions to the helpful advice I've read above as to render all of it useless.

Titanium - I own one unpainted, one fully painted. One fat tube, one skinny tube.

Aluminum - One fat tube, several shaped tube, one skinny tube.

Steel - fat, skinny, shaped, etc.

CF - skinny tube and shaped tube.

The only way to uncover what the frame you're looking is made from is research (builder's sites, ask here, etc.) Steel - sure, bring a magnet along, that will help you 25% of the time.