Harry Bloomfield wrote:

with no working boiler there is no easy way to work out which is the
flow, which is the return at the rads


Some TRVs (e.g. Myson 2-way) are marked with a double-headed arrow to
indicate they can be fitted either way round, maybe yours are too?

That said, I still try to fit them so it's flowing in via the TRV, just
in case it helps cut down on annoying whistling.

Can you tell by which end of the rads have the lock-shield valve being
the return, I suppose that's only a convention, no guarantee.