Some words you can get a feel for. Kiti/viti (chair(s)) and chakula/vyakula (food) are usually Ki/Vi nouns. Mti/miti (tree(s)) is M/Mi, etc. If you know the plural is the same as the singular you won't go far wrong with N/N.
I would also go and google Swahili noun class semantics to find out why words fall into certain groups. Having that information on hand is a huge help.
The tricky bit is remembering that animate nouns (animals, and people not already in the M/Wa class) follow their class' s pluralisation rules, but otherwise follow M/Wa grammar. Daktari(N/N noun) wangu(M/Wa concord).