Assuming it's a quill stem, although all 1" steerer tubes all have a 25.4mm O.D., the wall thickness varies so that the I.D. of the steerer tube also varies. Most quill stems have a 22.2mm diameter, but some older road bikes and even more recent MTBs have thicker walled steerers and take 21.1mm diameter quill stems.

So no, it's not a slam dunk that any 1" quill stem will fit your bike. Your best bet is to either look for the diameter stamped in your stem, or use a caliper to measure it.