of course it's impossible to know 100%, but being a 1 in 255 chance plus the checksum and other marker bytes, it's pretty safe.
As long as you don't get out of sync with the incoming data in which case it's possible you will never recover without taking further measures.
Statistics aren't my strong point but I would say way it's less than 1 in 255, as any byte can == the start byte I think it's 1 in (255/frame length).
Is this data continuous or is there large gaps between the frames?
Rob