To simply answer your question the only way to prove the beats are yours is the software files and legal signed documents if you have copyrights towards your beats.
I've been prodding for over 4 years and I have never been questioned if my beats were really mine and thats probably because my prices have always remained the same. My guess is your prices are to low and thats why someone is raising their eye brow and asking themselves 'whats the catch?' Im not sure what your prices are but basic leases shouldn't be anything under $10.