In California, for a partime employee, if you are asked to work, is
there a minimum number of hours you should be paid for...for example 2
hrs? I have been asked to work for an hour and a half period....and
where can I find this in writing to show my employer?

http://www.dir.ca.gov/IWC/WageOrderIndustries.htm
The above is the site to download the various wage orders for the
various industries (mfg, personal service , transportation etc...)

5. Reporting Time Pay

(A) Each workday an employee is required to report for work and does
report, but is not put to work or is furnished less than half said
employee's usual or scheduled day's work, the employee shall be paid
for half the usual or scheduled day's work, but in no event for less
than two (2) hours nor more than four (4) hours, at the employee's
regular rate of pay, which shall not be less than the minimum wage.

I think the phrase "usual or scheduled day's work" includes usual so
it may apply in  your case (assuming you are not exempt)

You can also call the DLSE district offices the numbers can be found at
http://www.dir.ca.gov/dlse/districtoffices.htm

Be careful as fallowing through with this may put you on your
employers "hit list" evan though retaliation is against the law.(there
are time limits on both filing a wage and retaliation claim.}
Hope this helps