I would use HTTP Lib for this task (http://www.basic4ppc.com/android/help/http.html). Init with the Url of your file and then use StatusCode As Int [read only]. When the Code is 200 the file will exist. I didn't tried this, it is just an idea how to start. may be there more easy or better solutions...
Another workaround is to download the file in a try catch and then check if the file exists locally by using a timer.
Would be rather effective for smaller files. Perhaps not the best solution but a working alternative.
KMatle posted a very usefull tutorial how to get a list of files from a directory on your server via php.
i use it in my apps and its working great (i have modified a little the php file to get more details back)