Actually I find Bufo species, at least the ones I have experience with (from North America and Europe), to be amongst the easiest of anurans to sex. Males tend to be skinnier and slightly smaller (or in some species a lot smaller). They also tend to be more brightly coloured than the females, though this can be as subtle as a better contrast between the animal's colours. Some species, such as American toads, have differently coloured sexes - males are tan/brown, and females are a brown or brownish-red.

If you need confirmation, put your fingers behind the front legs, simulating amplexus. This usually causes the frogs to squeek (their release call), if done right. Females are usually reluctant to squeek for humans doing this, so that helps. Also, if you look at the throat as you're doing this, the throat sac of a male is usually apparent as a flabby area on the throat (absent in females), and in some species of Bufo the sac is dark in colouration - if you have a female present too then you can compare - it is usually very apparent.

Lastly, and this isn't as easy if you're looking for toads in the wild, particularly at night, males have slightly enlarged thumbs to help better grip the female - in the breeding season they have pads on the thumbs, and these are usually dark in colour, and obviously the pads and the colour is absent in females.

Bufo in general are hard to breed unless you can give them a good cool period and provide the right stimuli when breeding season comes around. You can still keep them together - it makes no difference. If you can get them to breed, depending on the species they can lay from a few hundred to over 50,000 eggs.