Some teak substitutes have very little figure in the grain, but teak is variable so this is not foolproof. The genuine article has a distinct smell. Gilly Goulden would say leather with a peppery afterglow. The surface of the wood should feel waxy to the touch esp if you take a shaving with a plane.
You may be lucky, two years ago I saw some shocking quality (and cheap) garden furniture made out of the stuff
The standard test is to put a piece in a skip and measure how long it takes to disappear, if in nanoseconds then the wood is teak.
Seriously, the best way is to compare against various bits of wood that you know to be teak remembering of course that wood is a natural product and varies.
If you visit your local wood yard they usually sell small offcuts that you can use for comparison - they might even give you a few sq inches. I was in my local wood yard today and remembered your question - but I still cannot describe how teak looks in words!!
Many libraries keep a book on wood types that has small pieces of veneer for comparison.
I had a craftsman make a new table top for my saloon. He didn't use teak but one of the cheaper alternatives. It matches everything else on board perfectly, although whether everything else was actually teak or some cheaper alternative, I could not say.
Within the range of what may appropriately called teak, the qualities w.r.t. colour, grain, durability, etc. vary enormously.
There are other kinds of wood having similar or better qualities for use on the boat (insode/outside) that are definitely not wood.
If it is hardwood and looks like what you want - why care ?
Practical test: take 2 pices and put one in a tidal area and the other onto your roof - check around X-mas whether they are fine (and/or can be easily refreshened with some light sanding)
If you are still curious - teak has a very special odour and you should be able to smell the difference vs. a known piece of teak
As said earlier, Teak has a distinct smell, I picked up some Lab Benches which had been chucked in the skip, all solid Teak, although with various holes etc in them, some 16' long, made some fabulous worktops for the kitchen complete with genuine 'patina' - but discovered one strip was a 'mahogany type' wood after final cutting to size & sanding - not much of a problem and you have to know it's there to notice it. I now have lots of small offcuts for the boat! If you want a small sample I can post to you, PM me if this will be useful. As the benches were about 60 yrs old it is almost certainly genuine Burma Teak, not the plantation grown stuff you get nowadays.