IF YOU PLAN to watch the various competitions at the Scottish Games, it helps to know who's who and what's what.

Among the approximately 25 professional athletes are several of note:

*Edward R. McComas, 52, a Johns Hopkins graduate from Lutherville and president of his family's home heating oil business, has competed in more than 120 games and won 59 of them, more than any other athlete. He held the record for the sheaf toss until last June. So respected is McComas that a trophy is named for him.

*Keith Tice of Clovis, Calif., who teaches junior high school, runs a eucalyptus tree farm and makes hot tubs, holds the American records in both the 28-pound and the 56-pound weight throws and the 16-pound and 22-pound hammer throw.

*Jim McGoldrick, a bank loan officer from Cupertino, Calif., is waging a head-to-head competition with Tice to break records.

*Michael Reid of Monkton, Md., a cabinetmaker specializing in 18th-century reproduction furniture, is the smallest of the professional athletes, at 5' 10" and 175 pounds.

Some people to look for in the non-athletic competitions:

*John Turner of Chesterfield, Va., is considered the top American-born Scottish fiddler. Watch also for Steve Hickman, an Irish-style fiddler who has his own Scottish country dance band, and 12-year-old Stuart Ian Duncan from Vista, Calif., who has fiddled in his father's bluegrass bands.

*Cheryl Fisher of Bowie, a student at Ursinus College, who has competed in all five national Scottish dance championships and last year was named national Champion of Champions.

*During the performances of the massed bands (22 corps in all) at 12:30 both days, look especially for the Lionhart, the City of Alexandria Pipe and Drum Corps, and the Scottish and Irish Imports Pipe Band of Annapolis, whose 25 members wear the Black Stewart tartan. The last group has won most major prizes for pipe bands on the East Coast, including the Eastern U.S. Pipe Band Championships two years ago.

A couple other items of interest:

*A nice spot to relax is the wooded glade on the far side of the Episcopal High School fieldhouse, where you can drop in on the Third Annual International Scottish Harp Competition, meant to promote interest in the ancient Celtic harp and in Scottish song.

*For an understanding of the history of the games, you might pick up Emily Ann Donaldson's newly published "The Scottish Highland Games in America" (Pelican Publishing Co., $19.95). She'll be around to chat and autograph it both days from 10 to 2.