The Acers have palmate leaves and the Sambucus have pinnate leaves.

Or you could put a photo up here and someone will identify it for you.

Google 'Sambucus nigra' and click on Images on the left when the page comes up. Do the same for Acer palmatum...assuming that's the variety you want to check it against.

There is a bush in a local park that I took to be an Acer palmatum dissectum but only realised recently that it is a type of Sambucus nigra lace. The two do look really similar in full leaf.

As Roseberry has said, it is to do with the shape of the leaves. Acers leaves palm out and Sambucus are in a line.

I thought that this was a pretty strange question until I read Kildermorie's answere, sorry Spendersgarden for any offence. I think that your best bet would be to add a photo on here, otherwise google for the images of both as suggested by Bamboo. ( Three GoYers named in one short answere - wow)

I think most people think of Acer Palmatum, which has one fat leaf like other maples. The dissectum has the cut leaf that looks most like Sambucus nigra lace varieties.

The other way to spot the difference is location. Acer Palmatum dissectum does not do well is exposed and sunny sites while Sambucus nigra is fine and grows large in it and vice versa. My local park has a Sambucus nigra in a shaded, sheltered spot and while beautiful it is quite dwarf while the near by Acers are trees.

Thanks to all. Yes, in my opinion the two are easily confused -sorry I didn't give full name of acer when posting question. I was thinking of the palmatum dissectum.
I have learned now what pinnate is - a new term for me, though I knew palmate.
I went to take a photo and check the leaves (the plant is at my son's house) and saw that it was flowering so that settles it. Some of the sample leaves I brought away do still have a hand-like shape but are definitely pinnate. The photo I took confirms pinnate (can't seem to upload it as necessary bit re choosing file is greyed out (?) am on iPad so maybe that's the problem.
Anyway, thanks a lot.

My problem, Kildermorie, is that I am familiar with many acers but not so with Sambucus.

The easiest way to tell is to crush a leaf between your fingers and smell them. A Sambucus will smell very strong and a mixture of sharp and musty - an elderberry smell! I don't think an acer would smell of anything much.