There are a couple tricks you can use to make it easy to keep track of loans that are maturing soon:
The red exclamation-triangles along the left side of the list of loans will light up when loans are getting close to maturity. Hover or click them to see a message about the exact date of maturity. You can control how many days beforehand this happens from Portfolio Settings > Numbers tab > "show the maturity warning..." Default is 65 days.
Add the maturity date column to the main window. File > Moneylender Settings > Columns tab > add "LoanLastPaymentDate" to the Displayed Columns. You can sort on this column to order the loans by which ones mature the soonest.