You can verify the GST invoice by visiting the official GST, portal https://www.gst.gov.in/and verifying the GSTIN
On the homepage, select 'Search Taxpayer' to check the GSTIN number given in the challan. If the GSTIN is genuine, you will get its details on the website.
If you know the meaning of the 15 digit GSTIN number, you can identify a fake GST bill.
The first two digits of GSTIN represent the state code, the next 10 digits are the PAN number of the seller or supplier. The 13th digit is the unit number of the same PAN holder, the 14th digit is the letter 'Z', and the 15th digit is the 'checksum digit'.