I've done a lot of customer development in this market.
Here's what's worked for me well. Find your potential customer (actual decision maker) on Linkedin.
Send them a "friend" connection request with something like:
"I acknowledge we don't know each other yet but I am the Founder of a new company solving <insert problem>. I'm not trying to sell you anything just would like to ask a few questions to determine if we're on the right path. Would that be ok?"
That might be a bit too verbose for LinkedIn's character count but you get the gist. In the times I've used this approach, I've received about a 35% success rate.
I've done a lot of ideation around SMB-based ideas so happy to talk in a call about how to get the most out of your early conversations and experiments.
The best way to validate an idea is to go sell it. Basically in alignment with what Tom said, set up conversations and see if that is what they want. Look at this as research above and beyond everything. Have as many calls as you can to get a better idea of what people really want.
I've done a lot of work in sales around creating opportunities to get people on the phone and have real conversations. Happy to help you do the same. I know it can be scary!
SMB is a tough market, I've been working in for the past 4 years, on my startup BizeeBee.
However, you seem to have a really great focus: managing subscription based services.
Depending on the size of the SMB (more medium) there maybe one person managing the services, such as a business partner, accountant, or CFO. However, if it's a smaller business then it could be one person who is wearing multiple hats, and then it's a matter of convincing them to give you the time of day.
You might want to offer a little bit more a teaser, by addressing some of the problems SMBs experience when managing subscription based services, asking if the SMB you are talking to experiences it, and who the best person would be to talk to regarding the management of services.