I am using yaho mail to send or to receive importent information from
my Executives. How to know If any other person hacked or cracked my
mail account?
If nobody hacked/cracked it so far , how to know if any body tries for
hacking/cracking my account in future and how to get protected from
their attacks.

I would recommend changing your passowrd on a regular basis

If you usually read your mail in the same computer, check against
spyware and viruses also on regular basis. There are programs designed
to steal passwords.

Additionally, try to not use weak passwords like dictionary based
passwords since they're easier to guess. Try to use a combination of
caps, lower, numbers and signs such as #,$,!, etc.

If possible, try using a passphrase (I know not all webmail providers
allow such passphrases, but it makes life easier). It is much easier
to remember a passphrase like "My hometown is LA and I love SoCal!",
which is just as hard if not harder to crack than 48#4*Al(be or
something crazy like that which you'll never remember (and inevitably
have to write down, making it less secure).

If you are sending and receiving really important information I would
suggest you to take a look at PGP
http://www.pgp.com/products/index.html

I'd switch to HushMail:
http://www.hushmail.com/

Both the sender and receiver need to have an account --either free or
premium, then you can sleep easier...

First, make your password long enough (i.e, around 14 characters). And
change it every week. Second, all the data sent and received between
you and yahoo! is encrypted, because yahoo uses SSL (secure socket
layer) to protect the data from being read by other people on the
network. Finally, you can't know if someone is trying to hack your
account, because hacking methods vary and depending on the method the
result could be different. Always make sure that your computer doesnt
have keyboard logging software installed, else your password could be
sent to the person who installed this software. So if you are in an
Internet cafe, do not check your email there. Install Microsoft
AntiSpyware which is free. It protects you from most of the worms on
the Internet. Especially the ones that send information about your
computer.

By default you can't with yahoo....

There are other ways you can get a fairly reasonable idea whether
someone has tried to open your account while you were away.

1. Note down the number of new messages in the yahoo main page 

it shows something like this

Inbox(26), Bulk(7)

2. Next time before you login to your account

Log in to yahoo messenger with the same id.

yahoo messenger pops out a message / tooltip saying

"you have 10 new messages"

3. now log in to your yahoo mail address
the no of mails should read 10+the number of mails last time you saw
before logging out last time
in our case

it should read:

Inbox(36), Bulk(7)
if there are more or less...then there is a very strong chance that
someone has tried to open your account




Things you need to do on your computer

1. Use the latest browser preferably mozilla firefox 1.0.6 at the time of writing

2. Donot use mail account from public places

3. Ensure you have the latest antivirus updates

4. Check for spyware using Ad-Aware/ your antivirus or any other new
anti spyware tool

5. put a decent firewall on you computer..if nothing use Zone Alarm

6. MOST IMPORTANT: if you are accessing yahoo through web

use ssl     goto   https://login.yahoo.com
  nOT repeat not   http://login.yahoo.com

see the difference http and https.

And finally enable your bulk mail folder and donot open spam or suspicious mail
Logout and close your browser