The Executive branch checks the Supreme Court because decisions often require presidential action. For example, a Supreme Court ruling about federal stem cell funding will require the president act on the NSF. Also, the president appoints justices for Senatorial approval.
The Legislative branch checks the Supreme Court because it controls funding and can amend the constitution. Since the Supreme Court must oblige a constitution, an amendment is out of its reach. In addition, the size of the Supreme Court may be adjusted. In response to his New Deal policies being consistently rejected, President Roosevelt wanted the Supreme Court increased to 15 justices. This would allow him to appoint 6 additional justices of his preference.