You can add QRCODES to your permit and emails for anyone armed with a free QRcode Reader App on their smart phone to check. This can validate the qrcode to see if it is a real and active booking.
Add the following tag to your templates (including email notification templates). You can use the tag $(qrcode: xxxx) in order to encode any information in any format, e.g. $(qrcode:$(details_url)) or $(qrcode:$(resource)-$(start_date)-$(end_date)) which would create a QR code with the resource name, and start and end dates, all separated by dash. You can use all the tags inside the $(qrcode:...) tag, including calculations and date/time formatting tags.