OpenSea shows collection statistics alongside every listing: floor price, volume over various periods, owner count, listed percentage. For casual use that answers most questions without leaving the page.
The unfiltered volume problem
Volume figures are reported as transacted, with no published adjustment for wash trading. During periods when incentives made self-dealing profitable, that difference was enormous on some collections. A marketplace has an interest in its collections looking active, which is not an accusation of manipulation — it is a reason to cross-check anything material against CryptoSlam's adjusted figures.
What is genuinely useful
Owner count relative to supply, listed percentage and recent sale history are all reliable and immediately available. Listed percentage in particular is informative: a collection with 20% of supply listed is telling you something about holder conviction.
Cost and coverage
Free, covering every collection OpenSea lists, which is the widest coverage available.
Who should use it
Everyone, as a first look. Verify volume figures elsewhere before acting on them.