The best way to asses scientific knowledge is by practice examination. The method I usually use is to teach and go over the required topic with the student answering any questions along the way and then giving them practice questions to ensure that the content was understood. I base my teaching on the exam board and curriculum of the student to narrow down the essential information.
In biology, I always find that drawing diagrams and certain process helps to retain that information in the students head. If the student is still unsure of the material I can assign specific videos that have proven to be very helpful in clearing up any content.
However since the sciences are difficult subject I tend to set a reward system based on the student to incentivise them to get the work down.
At the end of each module I also test the pupil on al the content we covered and go back over anything that seems to be misunderstood.

if the lesson is quite long the student can get a break to regain some energy and be able to focus on all that is being taught.