How to figure out whether a force mediator is $W^+$ or $W^-$ in Feynman diagram?

How to figure out whether a force mediator is w+ or w- in feynman diagram?

I always make sure each vertex is zero but sometimes I get my w+ or w- worng.

• You should just be labelling them as $W$ in any case. – jacob1729 May 22 '19 at 12:53
• Did you check for conservation of electric charge? – Qmechanic May 22 '19 at 14:46

You can't tell from a Feynman diagram whether an internal $$W$$ line represents a $$W^+$$ going one way, or a $$W^-$$ going the other way. This is actually one of the key benefits of Feynman diagrams. Older formalisms such as old-fashioned perturbation theory would have represented this as two separate diagrams, but the Feynman propagator accounts for both automatically. For more details, see Schwartz's QFT text.