forklifts can have an attachement with weighers on them so that you can accurately know the weight, or you could have a weighbridge/axle weigher to check weights, for not much money.
personally, the issue comes when you then know the vehicle is overweight and causing/permitting an overloaded vehicle to leave site, knowing it's over the limit.
the best rule of thumb for vans would be an absolute maximum of 1 tonne. given the 750 kgs limit currently being looked at by hse, that'd be a pallet and a few bags, plus driver (80kgs) and other stuff already in the van as a maximum.
liklihood is that they wouldn't get stopped, and unless there was an accident it wouldn't get looked at, but hse may get interested in the loading and control of loading, especially as you will probably have your own operators licence and committments to overloading within that, realistically you know there is a problem, because otherwise you wouldn't ask.
a disclaimer of weigh of pallet and no resposibility, with their signature in a book and sheet to them if they go over a pallet may be a good start, with some lawyer wording on their responsibilities, diligence, and instructions and/or offer of delivery to site refused as an option. with sign/print, date. time, reg & vehicle type/payload available estimate (filled in by them).
you could have a weighbridge facility at the gate for entry/exit and use for your own use to know you are ok, however the liklihood is traders have no interest unless previously they have had discussions with DVSA or hold an o-licence of their own.
DVSA and Traffic Commissioners are aware vans are where overloading takes place, but there's no appetite to reduce past above 3.5 tonnes for o-licences from the government, but dvsa will notice and pull a van "sitting on it's arse" and are getting fixed penalties on vans and associated costs against drivers.
The other issue is load security, and that will be something to address as part of the process too at the same time, other posts on the board discuss the issues, but load security is up the scale in terms of interest for DVSA.
once it's on the highway the driver is responsible, however the only real place where i have seen specifications of roles and responsibilities is within ADR, so if you want to look at that it'll give you the roles of loader, packer, driver, operator and receiver.
the senior traffic commissioner guidance on drivers, operators, and transport managers explains with some examples what is expected of a transport operator, and as such perhaps you want to consider how you would explain looking one way for collects, compared to own account operations, and how one would answer such questions, knowing the rules.
good luck
thanks