Just marking a shoe with ISI is not sufficient. The following points can be cross-checked to identify the genuineness of a shoe marked with ISI:
Check for marks on the shoe. Marking on the box is not enough.
Check for the authenticity of the CM/L number on the website of the Bureau of Indian Standard.
Ask the supplier to submit a test report from any government Lab for testing as per ISI.
If the consignment is big then the buyer may also submit the sample at FDDI, Noida for testing as per IS 15298 Standard. This involves some expense but if the purchase amount is significantly high then the buyer can choose to use this as the most effective option.
Take references from known sources.
Visit the factory of the manufacturer, if possible.