If you're talking about pterosaurs, the best evidence that they could fly is that they had lightweight hollow bones and keeled breastbones for anchoring powerful flight muscles, just like birds. These features would surely be impractical on land and serve no useful purpose.
from wikipedia:
Pterosaur - Wikipedia
quote:
One of the chief arguments against active pterosaur flight has been their relatively shallow sternum keel, which is the anchor point for the pectoralis muscles, the main flapping muscle. However, pterosaurs display other skeletal features that may have made this less problematic than a direct comparison to birds may indicate. The pterosaur group is notable for a unique bone, called the pteroid, in the forearm, which may have supported a flight structure not reproduced in other flying animals.
This suggests that the keel was as much an argument against flight as for it.
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