Function To Say If File Exists

I would like to have a user designed function that will tell me if a file exists in the specified directory.

For example, my file name is wd1.xlsm. I have another udf that tells me my current file path - e.g. C:All Sales People Quotes System.

I combine these two in say, cell D4 and so have C:All Sales People Quotes Systemwd1.xlms

Now I would like a udf that I can put in E4 that will tell me if wd1.xlsm exists in the C:All Sales People Quotes System directory.