What's the best way to approach them without insulting them? I've found some clients who I'd love to help but I'm not sure how I should politely and correctly word that their designs suck.
Before jumping in and telling them their website or app stinks, start a conversation that let's them talk about things from their perspective. Ask them how things are going - if their conversions are where they think it should be; if their users are happy, why or why not; what they're doing about it; and what they would love their website/app to do for their business.
Get them to start thinking how things can be better, start understanding their business goals, and how design can improve those goals, and they'll probably sell your services to themselves for you.