Specifically in German legislation: how would they have to phrase the question so that they don't get in conflict with the law?
There is no problem with the law no matter how you phrase it. Even a direct "I don't believe you are sick, please bring your doctor's note on Tuesday" would not break any law (assuming the employee has a contract that requires him to bring a note or you have sufficient indication that he might be faking it). For any non-German reading this, please remember Germany has no concept of "sick day allowance". You are sick when a doctor says you're sick. Going to the doctor and getting such a note is free (well, already included in your mandatory health insurance). And the employer is allowed to ask for proof in form of a doctors note, most put it into their contracts.
Please note that "strictly speaking not illegal" might not be the level you want to communicate with employees. Personally, I would feel very offended if someone thought I'd be faking it. So being nice and respectful certainly does not hurt.
You may also not want to disturb a sick employee by anything intrusive like a phone call. This is not an emergency, neither for the company nor for him.
Send him a message, maybe EMail or SMS if you have his cell number. I will shamelessly plagiarize the existing answer and modify it a bit:
Hello Eric. Sorry to hear you feel sick, I hope you get well soon. Tomorrow is the conference. If you cannot make it, we need to send a replacement and they need to know before they leave the office today at 16:30. So please contact us before 16:30, otherwise we will have to send somebody in your place.
The point here is to set a clean limit. It's not arbitrary, it's a deadline for the business requirement that the trip has to be scheduled and the person needs to plan for the following day. But it leaves no room for interpretation what happens if the employee does not communicate (because if they are sick and maybe went to bed or took a lot of painkillers, not communicating would be the default).