You do not need to understand web development to hire well. You need five questions, and you need to watch whether the answers come easily.
You don't need to understand hosting or SEO to hire a web designer well. You need five questions — and more importantly, you need to notice whether the answers come easily or whether the person starts hedging.
1. Who owns the domain?
The only correct answer is "you do, and it's registered in your name from day one."
If a company registers your domain under their own account, leaving them becomes a negotiation. This happens more than it should, and it's usually not malice — it's just easier for them at setup. Ask anyway, and ask to see the registrar login.
2. What happens in month six?
Everyone is attentive before launch. The question is what happens when you need the hours changed in February.
Is support included, or billed hourly? Who actually answers — the person who built it, or a ticket queue? A cheap build with no support often costs more by the end of year one than a slightly dearer one that includes it.
3. What isn't included?
Ad spend, stock photography, domain renewal, third-party software. Every project has exclusions and there's nothing wrong with that — but they should be written down before you pay anything.
If someone can't tell you what's excluded, they haven't thought about it, and you'll find out by invoice.
4. Can I see something you built two years ago?
Anyone can show you a site they launched last month. The interesting question is what their work looks like after it's been lived in.
Is it still up? Still fast? Has the content been kept current, or has it visibly ossified? That tells you what happens after the invoice clears.
5. What would you tell me not to spend money on?
The most useful question on this list.
Anyone who answers "nothing, you need all of it" is selling. Someone who says "honestly, your site is fine — your Google profile is the problem" is worth listening to, even if it means a smaller job for them.
The thing underneath all five
You're not testing technical knowledge. You're testing whether they'll tell you something inconvenient.
A website is a relationship that lasts years. The person who's straight with you in the first conversation is usually straight with you in month eighteen, when it actually matters.