Can you tell the sex of a white willow when it's not flowering?
I remember overhearing a botanist some 10 years ago, that he cannot tell male poplar from female when they are not flowering or producing seeds. Poplars and willows are related, so it is a weak indication for you, you will likely not find any visible features in willow neither. But it is an old information, too.    (Are you interested in DNA sequencing evidence or not ? I can look it up for you, but it seems your statement in comment question was ironical.)
EDIT: have just found an article which says: &quot;In Salix viminalis, the sex of plants cannot be confirmed until the first time they flower.&quot; At that time, 1997, there were no DNA markers for plant sex determination, but it is apparent, they did not know how to differentiate the sex of this willow just from morphology. This example is even closer to Salix alba than poplar. I find it even more likely now, that you will not find the morphological features.
I took the suggestion of Oreotrephes. This is what to look for under the tree - fruit and seeds:
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Source.
( C. Alström-Rapaport, M. Lascoux, U. Gullberg. Sex determination and sex ratio in the dioecious shrub Salix viminalis L. Theoretical and Applied Genetics 1997, Volume 94, Issue 3-4, pp 493-497).