There's no one correct answer to that question.
Basically if the species are not similar in appearance then you could probably tell visually (fry sired by a yellow top mbamba and mouthbrooded by a yellow lab could probably be identified as being neither species). With similar looking fry you might not know until they're bigger (fry sired by a johanni and a mouth brooded by a msobo will resemble fry of either species until they're well-sized juvies, as fry of both species are born yellow-orange).
A decent gender ratio will greatly minimize the chance that hybridization will occur at all (1 male per several females per species is effective as most will seek out their own species first, then any willing participant second).
The chance of hybridization can be even further reduced by avoiding similar looking mbuna's, or avoid keeping dimorphic species with similar looking genders, or better yet: no more than one genus of mbuna per tank.
Depending on factors the mother will regard any fry she releases as her's, hybrid or not. Aside from temporarily reclaiming them in her mouth at the sign of trouble, I've never really witnessed any greater extent of protection from mbuna mothers (unlike my female nimbochromis venustus', they become ferocious to all other fish in the tank anywhere near their fry, they aggressively protect them).
There should be no real hybridization issue if keeping, say, decent sized groups of yellow labs, snow white socolofi and red zebras together.