You've posted to the FileMaker Go forum but make no mention of FileMaker GO in your post. Did you post here by mistake? (It's quite easy to do.)
IF you have a date field (must be of type date, not text), you can perform a find for all records where the due date is within 30 days of today if you perform a script like this:
Enter Find mode //clear the pause check box
Set field [YourTable::PaymentDueField ; "<" & Get ( CurrentDate ) + 30 ]
Set Error capture [on] //keeps dialog from interrupting this script if no records are found
Any records found will be records with paymnts less than 30 days in the future, this will include dates in the past, but I am assuming that the date in this field will be updated to be empty or show a new date as payments are recorded. If this is not the case, the script may need an additional set field step to specify criteria in a second field that indicates whether or not the record is "paid" so it can filter out those records.
Once you can get that script to work, you can take the next step to use it in a larger script that is performed automatically. (There are several ways to do this.) And it can then display a message, if so desired, telling the user that unpaid records in this date range have been found.
A bit more power can be derived by adding a calculated field:
if(duedate - currentdate <= 30; 1; 0 )
Now you just search for this calcfied = 1.
The extra power comes from being able to do a summary on this field and show a number telling you how many such payments exist.
Soon Summaryfield = total this calc field.
You can also create calc fiels for dating the amounts due: 30 days late, 60 days late, etc.
With this information in your toolkit you can really go crazy...stop....
...and this number field can also be displayed as a checkbox, valulist = 1, on your layout....