How can I check that my RDF document conforms to the spec?
W3C offers an RDF validation service; much like the Markup validation service, you point it at your document and it tells you that it's good or why not.
How can I check that my RDF document conforms to an RDF schema?
see RDF Validator, May 21 2002 from TimBL to www-rdf-interest, for one view.
an OWL checking tool can help. e.g. this OWL validator from folks at BBN, based on a DAML validator
try VRP - validating RDF parser, though it seems disagree with the specs in some ways
Eyeball is a Jena contrib for checking RDF models (including OWL) for common problems.
Are there any online OWL-DL checkers?
MindSwap operates a pellet demo. It rocks.
MikeDean of BBN operates an OWL validator somewhere... or did at one time. LinkMe.