Proof of true team bikes may be very difficult to obtain, since many such bikes were made in multiple examples and, of course, in most cases, the groups, saddles, etc., they sported were also usually made in series. In fact, several bikes may have been used by a given rider for a single event. Engraving, special decals, etc., may be helpful, but these can be reproduced.
The first form of evidence of a true "team bike" would be congruence with known photos or other established examples of this bike-lacking this, proof would be more difficult.
This would be followed by some form of provenance, e.g. a bill of sale, letter, etc.,with a hierarchy of such provenance extending from the bike's rider to the manufacturer, seller, etc.
George Hollenberg MD
CT, USA