The issue, in Belize and elsewhere, is rarely with immigration in the destination country but with the airline that you fly. Quite often (though not always) an airline agent checking you in or checking your passport will require a roundtrip or onward air ticket, or some other proof of onward travel, as the agent is following the letter of the rules of Belize or other destination country.
Again, this does not happen every time you fly, but it happens frequently enough that if you look at various travel forums and boards you will find quite a few examples of passengers who have been denied boarding or who have had to buy a refundable onward/return ticket before they were allowed to board.
This has happened in fact to members of my own family, though not involving travel to Belize -- in one case it was to Hong Kong and in another to Budapest. You can ask the airline you're flying for their policy, but that doesn't guarantee anything.
But to be clear ... over the past 23 or 24 years I've entered Belize by air, land and boat probably 50 times and have never once been asked by Belize immigration to either show an onward/return ticket or demonstrate proof of funds.
(Of course, if you are a citizen or an official resident or have other legal status in the destination country, none of this applies.)
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