How to find out if food service is available on train - Munich to Fussen

That is a regional train. I don't think there is any food service.

If you use the Bahn website, after you make a query and get some connections, you can get details for any single connection by clicking where is says, e.g.,
München Hbf Gl.27-36
Füssen
in the column "Station/Stop". Or you can click on "details for all". That will give you a list of all the trains for each conection, with some details. Under comments, it should tell you IF there IS food service. I don't find any of these trains that do.

For multiple train connections, details also shows you the time of arrival and departure at each intermediate station, as well as the track numbers for arrival and departure. If you click on the underlined train number, it will show you the complete routing of that train.

We've often purchased quick "picnic" foods at a local market, and eaten a quick meal on a train without meal service.

Admittedly, when I have been around the Hbf in Munich, I have not been looking for a "local market", but I think you would be hard pressed to find one. However, in the Hbf, just beyond the end of the tracks, there are a lot of shops selling premade cold sandwiches. I'm sure you could find something there to take with you on the train.

There is no food available on RE trains. You'd have to buy a snack or whatever at the train stations (there are always several options). Or buy your picknick at an Aldi or Lidl and consume on bord.

i remember nov 05 that munich hbf had lots of good looking food at not that much alas the euro was less then a dollar .. so now just lots of good looking food i guess.

I agree with Kelli...bring a picnic! Rick Steves' also recommends doing the same thing. He should know ;) It would be cheaper and more nutritious as well.