Schamroth's window test
Normal people will have a diamond shape gap between 2 fingers or toes as in A. If there is no gap as in B, it means the finger nails are curved, or nail clubbing.
If you have clubbed fingers, it is best to see a doctor. While it may be easy to panic, clubbed fingers often run in families.
You may want to read
1) U.K. woman’s curved nail post prompts lung cancer diagnosis
2) The simple 'finger trick' that could 'diagnose' lung cancer