How To Check The Ovulation Date Having Irregular Periods?

hi , my periods are irregular for past 5-6 month . starting from october, in october my period came on 6th oct ,then 15th november ,then 28th december .. no period in january .. now its feb 14th it dint came . i m planning for baby .but the home pregnancy test came negative .i cecked on 13 feb 2011. also to mention i have UTI 10 to 15 days before. due to my irregular periods i dont knw wen i ovulute. i m taking folic acid 5mg for the past 2month .