The answer is not easily. If you ask most experienced horse people, you'll find almost everyone has woken up to one more horse than they thought they had.
You can't always tell just by looking at her belly. Maiden mares (first pregnancy) often don't actually drop their belly that much - most cases of surprise foals involve maidens. A well conformed mare will already have a slight drop. In any case, mares don't "show" pregnancy until about the eighth month. Mares that have been bred a lot often don't "get their figure back," especially if they're older.
In general, you can't tell if a mare is pregnant until she's within a couple of months of giving birth.
The signs to look for are:
A slightly distended belly.
Visible movement in the belly - sometimes if the foal shifts you can see it. And it's also possible to feel the foal kicking if you rest a hand on the mare's belly.
Two to four weeks before the mare is due her udder will increase in size. One week before, the teats will increase in size and start to look waxy.
A pregnant mare may also lose endurance and tire more easily. She can get cranky in the last couple of weeks - probably tired of carrying around that extra foal weight - and many mares refuse to have their belly groomed or touched. Colic like symptoms in a mare who otherwise eats, drinks, defecates and urinates normally can be a sign she's about to go into labor - and can sometimes be the first sign you get. Also, there's a testosterone peak about 90 days into pregnancy which can cause a mare to act "studdy" and try to mount other mares. (This is sometimes mistaken for heat behavior). The sex of the foal doesn't seem to affect this.
But it can be really hard to tell - as most horsemen will verify.