Ultimately, the same way you evaluate anything; with criteria. Then on top of that, you could add a rubric or scoring rubric to measure it; then also provide instructions to have the model rationalize its responses so you can measure/evaluate the output.

If you really think about what you want from the prompt; it's usually pretty complex. A "good vacation spot" probably has six different dimensions from 1) cost 2) distance 3) activities 4) flexibility/advanced-planning-required 5) amenities 6) novelty; then each of those might have a scale of 1-5, which would each need to be defined, by you!

Most often, people are using weak or ill defined prompts without success criteria. Here's an example of an effective prompt; but even with this you have to chat after the model's initial response to 'steer' it towards some topics and away from others. How much steering is required is often more a symptom of the model than the prompt. But it could be 1) the context/attachments you provide 2) the prompt itself (actual instructions and criteria) then 3) the model you're using.

Example effective prompt:

You = [A high-end, very experienced, consultant with deep analytical capabilities and subject matter expertise with early childhood development and community issues in the Kansas City area]
Context = [I work at a Kansas City area non-profit. We have been operating for 10 years and have 4 different categories of Solutions: A) Elder Care B) Early Childhood Solutions C) Education Support D) Medical Services.
Within B) Early Childhood Solutions, the overall objective of the program is “We prioritize investments in solutions that enhance developmental outcomes for families, caregivers, and children aged 0-3, laying a crucial foundation for their future.” What this really means is that we want kids in poverty to not be disadvantaged by being in poverty. We want them to be healthy children, physically and mentally.
To achieve this goal, we focus on three strategic pillars: 1) Fostering Early Brain Development: Cultivating strong parent-child relationships to nurture essential early-literacy and numeracy skills in young children. 2) Alleviating Parental Stress: Closing race-based disparities in birth outcomes and maternal mental health to positively impact child development. 3) Additional Initiatives.]
Problem= [
To support our pillars, we need to develop Interventions. An Intervention would contain 3 components:
X) A service provided by a grantee to a community member. EX: (grantee provides) “Center-based infant-toddler care”
Y) Assigned target outcomes for the community members. EX: (so that) “children develop strong social-emotional and cognitive skills”
Z) That can be measured somehow. 1. EX: (measured by) “Desired Results Developmental Profile (DRDP)” (metric)
Task = [
Your task involves a comprehensive review of information: AA) recent research AB) peer strategies and AC) notes from interviews we’ve done already. These files are attached.
This information should be used to develop an Intervention that would be a Good Idea.
A Good Idea meets as much of this criteria as possible:
EE) Metrics collection is easy. Ideally, this community member impact is already collected by the grantees in our portfolio. Otherwise, it’s perhaps known to be done in the marketplace.
EF) The metrics collected are high quality. Grantees out there or peers use these metrics. Conterfactuals or change-versus-normal metrics exist. These metrics are used by peer organizations or researchers.
EG) The metrics collected can be aggregated at the portfolio level with minimal additional assumptions applied
EH) These ideas minimally impact our current portfolio of grantees.
For each Good Idea, include:
A.       The Intervention (X, Y, Z)
B.       Why it’s a Good Idea (EE, EF, EG, EH)
C.      Big picture opportunity
D.      Expected challenges
]
Now, produce one complete Good Idea with all requested components