Investigating a washing-machine drain that occasionally backs up

The washing machine drains normally most days, but occasionally the standpipe backs up and splashes water onto the floor during the fast drain phase. It doesn't happen when I run a nearby sink. I'm worried about a partial clog or an undersized drain connection rather than the machine itself. What observations would help a plumber distinguish a blockage from a plumbing-capacity problem? I don't want another surprise puddle.