If you're worried about getting them confused, just wait until about 24 hours later to see what kind of CM you've got. Checking several times a day is best (because it can change over the day), but once is enough. By then everything else should be gone.
Are you seeing LOTS of stuff that could be called EWCM, like every day, and that's why you think it might be the Pre-Seed confusing you? If so I'd say wait longer after use to check. But in my experience, when anything (lube or otherwise) is "left over" it comes out much waterier than it went in. So I'd be more worried about confusing it for watery than for eggwhite. Unless you're checking CM like right after BD. Which I don't think you'd be doing anyway.