Guppies IMO are the easiest to sex. Females can have a
little color, but it's hardly anything to notice (it's usually like a black or yellow color on a tiny section of their tail). Males usually have brilliant colors (yes, more than one), long, flowing tails, and patterns to their color. Unless your fish are spoiled & fat, you can look down into the tank at them - if their bellies stick waaay out from their sides, congratulations, you're having fish

. Also, when it gets close to time for her to give birth, their abdomen/stomach/belly area gets darker than normal. You can buy these little things that hang over the side of the tank into the water, but have netting around a frame, so that the fry can live in there safely. In my experience (I've had guppies that have
NO problem reproducing for about 4 years now), as long as you provide lots of leafy plants (live or artificial) for the fry to hide in, and as long as you feed your adults regularly (so they're not hungry & tempted), your fry will live. Also, guppies can give birth every 3-4 weeks, so in a few months, your adults will be used to having all the babies swimming about & won't bother them as much (at least mine did).