The reputable sites have independent auditors check their systems for fairness and randomness. This (for example) is the certificate displayed on Titan`s website:
RANDOM NUMBER GENERATOR
Titan Poker's software, developed and maintained by Playtech, uses a true random number generator to ensure total game integrity.
Playtech's gaming software carries an official Certificate of RNG Evaluation from Technical Systems Testing (TST), an internationally recognized and respected Accredited Testing Facility (ATF). TST has been working with industry operators, suppliers, manufactures and regulators to ensure that gaming products operate in a manner that is fair, secure and auditable and comply with some of the world's most stringent and comprehensive legislative and regulatory requirements.
TST laboratories provide fully independent and impartial testing and certifications for industry operators, suppliers, manufactures and regulators. TST analyzes system software and equipment, and the environments in which they operate, against manfacturer's specifications, legislative and regulatory requirements, and to generally accepted industry standards.
TST's services are delivered by a multi-disciplined team of trained computer scientists, engineers, mathematicians and information systems auditors, possessing first hand knowledge and experience in today's leading edge technologies. TST provides an impartial service that is renowned for its probity and fair dealing. It is a common belief that total impartiality of a testing facility is paramount in ensuring an unbiased evaluation.