you could use debug and release modes if you wanted to make life easy

i always use this code at form load (its in c#)
but you could use this to enable a bool that would disable some subrutines and just display a message that the feature is not aviable and to buy the real deal

#if (DEBUG)
// debug mode
this.Text = this.Text + " debug-" + this.ProductVersion;
#else
// Release mode
this.Text = this.Text + " " + this.ProductVersion;
#endif