What are the various ways to measure the effectiveness of an advertising/marketing campaign?

Marketing/Advertising campaigns are designed for different objectives and a measure of all these objective are never exact. It basically depends on the objective of the campaign. Digital campaigns are very easy to measure but conventional campaigns like televised, radio, on ground, billboarding etc campaigns are difficult to track and measure.

If your campaign is focussed at increasing conversion (sale, subscription, opt-in, survey etc) then the measure will be how many people reached the thank you page (in case of Digital campaign) or increase of sale/footfall (conventional media). These are easier to track but when a campaign objective is brand awareness, then it becomes difficult to measure the objective, we usually go for TRP or reach for televised, listnership for radio or podcast, or impressions for display ad, but in case of Billboardings & Postering it is a very difficult metrics.  

calculate if it was worth spending that money on advertising? some basic numbers are required to find out ROI that may answer your query.

I completely agree with Amarjeet's answer. I just wanted to add that efffectiveness of a campaign clearly depends on the objective. 

1) If objective is just sales - you can measure whether the campaign actually impacted the business by comparing the campaign period with the pre-period for the last couple of year after negating seasonality.

2) if the objective is just awareness - the reach, TRP, viewership, impressions etc. 

3) if the objective is conversion - you will have to take it up with the sales team based on questionnaires

4) if the objective is branding - very difficult to measure brand equity attained through particular campaigns.