I'm interested in a cyclocross bike and want to know what if any difference there is in fit to a road bike.What do I look for, is the top tube the same,the standover the same the head tube and so on. Thanks.

Depends what you want to do with it.

I use my CX bike about 90% on the road. I've got the same length TT as my road bike, and I run the bars about 5mm higher and use a 10mm shorter stem. If you want a pure CX race bike then this will change.

Generally speaking you'll have the CX bike setup shorter than your roadie (1cm closer, 1cm higher seams to be the norm), whether you choose to do this though top tube or stem would be your choice and would depend on your existing TT length / stem length, headtube heights etc.

The folks in the cyclocross section would be better equiped to answer this question.