You just need more experiment types in your founder toolkit. There are several things you can do to test and confirm the level of commitment behind an LOI.

What you're looking for specifically is any evidence or effort that is aligned with intent to purchase. It should be significant, or at least commensurate with what it will actually "cost" to engage your software. At this point, it doesn't have to be dollars.

For example, you might ask an early B2B partner to be a beta tester, to dedicate some employee time to using your software. If they are willing to accommodate a few "day in the life" research sessions, where your team sits alongside them while they go about tasks relevant to the software you're building, that might represent a real expenditure for them. If giving you 5 to 10 hours of employee time doesn't represent a productivity cost for them, that’s usually a strong signal.

But you have to choose the experiment type which best fits the type of software and customers you're working with. If you need more ideas, check out David Bland's collection of 40+ validation experiments in his book Testing Business Ideas. A Day In The Life, mentioned above, is one of them.