1. Actually standard IC50 will not be there for your plant extract if it is novel to explore.
2. For comparison you can choose any standard or reference drug (anticancer) and assessed it IC50 simultaneously with your plant extract.
3. By comparing these two you can have the idea about your plant extract that how much active it against a particular type of Cancerous cells when compared with a particular anticancer standard drug.
4. The similar or lesser the IC50 of your plant extract in comparison to standard drug shows its efficiency.
Agree with Nishat Fatima. MTT assay or any similar assays that measure the cell viability can be used to determine the IC50 values by using the extract at different concentrations versus control. It is always recommended to ascertain the IC50 values self with whatever cancer cell lines your planing to take your work forward than referring from somewhere else because a lot of uncertainties could be there when treating with crude extract.