This is a tested technique for people finding it difficult to talk to their SBs, because they're discounting the information they get as "making it up" or "their subconscious talking". It's been helpful for me, it always seems to work and it's helped me a lot in improving my relationship with Kirika (and other people in the system), so I thought I'd reproduce it here. This is only a technique for if you're having problems telling the difference - if you already talk to your SBs quite happily you probably won't need it, and attempting to use it would probably confuse matters. Also, it can make your SoulBonds' (and your) head hurt if you do it enough, or simply piss them off depending on what type of person they are. Decide for yourself if you think it's worth using or not.
Basically, if you hear your SB say something that sounds weird/out of place/out of character, or you're not sure if it's just you making up the words, try imagining them saying the opposite. If you get a feeling of "resistance", or like they're still insisting on the idea, then you know it's them talking and not you making it up. If you don't get any resistance, then either the words were in your head, or they're unclear about something, so try asking them to clarify what they're saying. If you don't get resistance or answer either way, and/or you find you can make them say quite silly things using your imagination without getting any "bad feedback" in your head about it, you're probably not talking to a SoulBond (or you're talking to a very lighthearted SB who doesn't really care - but testing this with Karl just now [guinea pig. ^^;], even then there's a little resistance). That doesn't necessarily mean you don't SoulBond that person, however, or that you're just a "soulpuppeteer" - they might just not be floating around close enough for you to talk to at that time, so your words are bouncing off of empty space.