Free tool
Hreflang Checker
Paste one page's hreflang block. It checks the three things that break nearly every set.
Hreflang Checker
Paste the block from one page's head. The checks that need every page's tags are noted as such.
Why it matters
Almost right means the wrong country sees the wrong page.
Three mistakes account for most broken hreflang. The first is a missing self-reference: every page in the set has to list itself as well as the others, and without it search engines drop the set entirely rather than partially.
The second is non-reciprocal return links. If your UK page points at the US page and the US page does not point back, the relationship is one-sided and is ignored. That check needs every page's tags, so this tool flags it rather than pretending to test it.
The third is the inversion: gb-en instead of en-gb. Language comes first, region second, and the reversed form is not a mild error — it is simply not a valid value.
Contact
Tell us the site and what it should be ranking for.
We will look at it and come back with what we would do and what it would cost. If we are not the right fit for it, we will say that instead.