Do they assess your property first?
Be cautious of a quotation given without any site visit or discussion of your roof and electricity usage. System recommendations that skip this step are essentially guesses.
Is the quotation itemised?
A clear quotation should specify the equipment brand and specification, system size, and what's included in the installation, not just a single lump-sum figure.
What happens after installation?
Ask what maintenance, repair, and warranty support looks like once the system is installed, and who you'd contact if something goes wrong. After-sales support is often where the difference between installers becomes most obvious.
Can they explain the trade-offs?
A good installer should be able to explain why they're recommending on-grid vs hybrid, a specific system size, or particular equipment, rather than pushing a single standard package regardless of your property.