third party checks

It depends.  Why did she sign the check over to you?  Did she owe you money, or was she just doing it for convenience so that you could cash the check and give the money to her?  There is a rule (the "Parole Evidence Rule") which might prevent your girlfriend from submitting any evidence to contradict what is written on the check (i.e. testifying that you were supposed to give the money back to her when it doesn't say that on the check), but I don't think it applies in this situation, where it is a small claim between two people who were in an intimate relationship, particularly if the check doesn't say what it is for.  If the check actually said "repayment of loan", for example, that would be a different story, and your girlfriend might be barred from submitting evidence to contradict that the check was signed over in repayment of a loan.