I guess my first question for you would be to ask further about what you mean by "low achievers." Do you mean students who have low grades in the class? Or do you mean students who are functioning lower than their peers academically or below grade level? Those would lead me to two separate answers.
To identify students who are behind grade level, there are a few options you can use. Academic screening measures are a viable option, but they vary depending on the grade level and subject you are looking to assess.
In a lot of schools nowadays, the focus is on identifying students who are behind in core subject areas so as to then provide them with targeted interventions to try to get them up to grade level. These assessments are typically quick and brief and target the core academic skill you're looking at improving (e.g., oral reading fluency, basic math facts, etc.). Two of the most commonly used ones are typically DIBELS and Aimsweb. Schools can invest in getting established screening measures for their school, but there are free options on the web as well. For basic academic skills, you could check out http://www.easycbm.com. They provide quick probes that you can use with individual students or with a whole class to determine who is falling behind based on established benchmarks.
If you are concerned about identifying whether or not a student actually has an academic deficit as opposed to a motivational deficit, you can do what's called a Can't Do/Won't Do Assessment. Basically this involves assessing the student on the skill at hand with (typically) three brief probes, and then challenging them to try to beat their median score by giving them an incentive. If the student drastically improves their score, you can get a sense as to whether it really is a motivation or an acquisition issue.