I have and old (what I believe to be) late 80's Pinarello frame I am rebuilding. The BB is an old Campy one that is shot. I would like to put in a Shimano one. How do I tell if it is Italian thread and what size I need. .

Apologies on the questions, This is my first time rebuilding anything old.

thanks

If it's a Pinarello, it's an Italian BB (70mm). If you have a Campy crank, don't use a Shimano BB, the square tapers are different.
The exact size would be the same as the one installed, as BB size/length are generally determined by the crank.

The exact model of crankset is the main determinant of what bottom bracket is best.

1980's Pinarello will of course be Italian-threaded, and Shimano does make a good variety of bb's with Italian threading, but the taper is slightly larger, so the cranks don't go quite so fully onto the spindle ends.
The original bb is also possibly offset, extending further out on the right end, so this will figure into the length requirement of any replacement.
Also your frame will likely have more or less chainring clearance than other frames, which may need to be respected if you use a different bb.
The final "chainline" (offset of the chainrings away from the center of the seat tube) can be biased inward somewhat in many cases, which can improve the running angle of the chain if you like to use the full range of rear cogs while using the large chainring. This is the way I prefer to set them up.

As you can by now tell, this can get complicated. Many will advise using only original parts, but I have many times been able to get a Shimano bottom bracket to fit well and work well over the long term. The trick for me has been doing a test-fit or two using available cartridge bb's that I have at hand, which allows me to home in on a minimal chainline dimension that works on a particular bike. It is also acceptable to use fixed-cup spacers to bias the chainline outward, if the chosen bb turns out to be on the short side, which may be the best for you as it often has been for me.
Since I never use the small chainring with the smallest cogs, I don't worry about having the chain running clear of the big ring when in the smallest front and rear sprockets, so a shorter-than-original spindle causes no problem for me.
Remember that a Shimano bb spindle will space the crankarms about 4mm further apart overall than will a same-length Campagnolo spindle, because of the thicker tapered sections. That translates to two mm per side.
There are also "ISO" bb's that have an even smaller taper, equivalent to about 3mm per side narrower than a Shimano spindle or 1mm per side narrower than a pre-cartridge Campagnolo bb.
A cartridge-style Campy bb will be ISO taper, so may act a couple of millimeters shorter than any earlier Campag bb of the same overall length.

Thanks dddd. I am planning to use some Shimano cranks with the shimano BB. Does that change things?

Sheldon Brown's Bicycle Glossary Bo--Bz

Sheldon Brown's Bottom Bracket Size Database

Square Taper Bottom Bracket Interchangeability

Campagnolo put the thread spec on their BB cups. If it's Italian thread, it should show "36 x 24 tpi"; if it's English, it should show "1.37 x 24 tpi"

Which cranks will you be using? If it's one of the Dura Ace 74xx-series cranks, the bb is spec'd at 113mm or 115mm.

They spec'd the wider 115mm bb for use with Italian frames, which makes no sense in terms of the chainline since Italian frames are no wider in back where the cogs are.
Maybe they are supposing that the Italian 70mm (vs. 68mm) bb shell might spread the chainstays apart enough to create a clearance problem with the small chainring, but I've even gone as short as 70x107mm on certain Italian-threaded frames with 74xx-series cranks and saw only benefit from the improved chainline.

Since the older Campagnolo bb's were offset toward the driveside anyway, I normally try for a shorter-than-spec'd Shimano cartridge bb with any Shimano crank, then have the option of adding 1-3mm of bb fixed-cup spacers to add any minimum of needed offset. I have the advantage of keeping various widths of bb's on hand for trial-and-error, and once a known bb is test-fitted, measurements can be taken to either home in on a better needed bb, or to add some appropriate width of fixed-cup spacer.
I like symmetry at the pedals, but 1 or 2mm offset at the pedals is well within the range that was once almost universal. The shorter bb of course also increases pedal-to-road clearance slightly, and is slightly lighter and stiffer in theory. I had to fix too many (even OEM!) builds that were just too conservative clearance-wise, and left the chainline so far out that using the big chainring and largest cog together sounded like some kind of metal-shredder, ...Not for me!