How to check if a file exists, and display a custom dialog box

I would like to make a macro that takes information from two cells (a date and a product number, for example) and generates a file name based on that information.

Then, the macro will save the file on a networked drive with the generated name.

Here is the functionality that i do not know how to do:
Before the file is saved, i would like the macro to check if a file by that name already exists. If it does exist, i would like the macro to alert the user and ask if they would like to overwrite the file (just like it normally would, except with some slightly altered language from the generic overwrite dialog box). any ideas?