Some toys of course don't work with a condom but for everything else that's non-porous and non-toxic that's the simplest thing. Against non-body safe toys condoms are probably not effective at all. I say probably because there's barely any science but you shouldn't have those kinds of toys anyway (think jelly dildos and such). Of course that's easier said than done because high quality toys tend to be more expensive but there are very good affordable silicone toys if you check with your favorite reviewers. Make sure that you don't use condoms with anything other than water based lube though and check if the condom's material is compatible with your toy.