Most supply teachers and teaching assistants pick an agency the same way: someone at work mentions a name, or an advert appears at the exact moment the current job stops working. Then they sign up, and hope.

It is worth ten minutes more thought. The agency you register with decides how often your phone rings, how far you drive, how you are paid and whether anyone knows your name when something goes wrong. Here is how to tell a good one from a busy one before you commit.

Ask how you will be paid, in plain terms

Start here, because it is the question people feel awkward asking and later wish they had.

• Are you paid PAYE by the agency, or through an umbrella company? Both exist in supply. If it is umbrella, ask what fees and deductions come out and who pays them, and ask to see it written down.
• What is the daily rate for the work you are being offered, and does it change for a long term booking?
• How does holiday pay work, and is it rolled up or accrued?
• What day do you get paid, and what happens if a timesheet is late?

A good consultant answers all four without hesitating. Vague answers about rates "depending on the school" tend to stay vague once you are working.

Find out who will actually ring you

Some agencies give you a named consultant. Others put you in a pool and whoever is free that morning calls you. The difference shows up in the quality of your bookings.

Ask directly: who is my consultant, and will it be the same person in six months? Then ask what they know about the schools they book you into. Have they been inside them? Do they know which ones have a strong behaviour policy, which SENCO will brief you properly before you meet a pupil? An agency that visits its schools can tell you that. One working from a spreadsheet cannot.

Check that they listen to your limits

At registration, tell them something specific and slightly inconvenient. That you cannot work Fridays, or that thirty minutes is your real driving limit. Then see what happens. If the first three calls ignore what you said, that is your answer and it will not improve. Good agencies write your preferences down and match to them, because a badly matched booking wastes their morning as well as yours.

Look at how they treat compliance

Registering properly takes effort. There is an enhanced DBS check with the children's barred list, references covering your recent work, proof of identity and qualifications, and a safeguarding conversation. If an agency seems keen to skip any of it to get you into a school on Monday, walk away. Schools notice, and it is your name on the sign in sheet.

The flip side is speed. A well run agency starts that paperwork the day you register rather than the day a booking appears. Ask how long it usually takes and what they need from you.

Read the reviews properly

Star ratings tell you little on their own. Read the words and look for two things. Reviews that mention working with the same agency for years are the strongest signal there is, because anyone can be pleasant for a fortnight. And reviews that thank a consultant by name usually mean one person looked after someone for a long time. Be wary of reviews about how quickly someone was placed and nothing about what happened afterwards.

Should you register with more than one?

Plenty of people do, particularly at the start, and it is a reasonable way to find out who works hardest for you. Two things to keep in mind if you go that way.

First, be straight with everyone. If you have taken a booking elsewhere for Thursday, say so on Wednesday. Being unreachable at 7.15am is remembered far longer than a polite no.

Second, agencies invest their best long term roles in the people they can rely on. If you are half available to four agencies, you may end up with day to day work from all of them and a long term role from none.

Questions worth asking before you sign anything

• Who is my consultant, and what is their direct number?
• Am I PAYE or umbrella, and what comes out of my pay?
• What time in the morning do you usually call?
• Do you visit the schools you book me into, and will I get a briefing before I arrive?
• What happens if a placement is not working out?
• Do you pay a referral bonus if I recommend a colleague?

The signs of a good agency, and a bad one

Good agencies are honest when they have nothing for you this week. They tell you the daily rate before you accept, not after. They brief you on the school. They ring to ask how a placement went. They remember that you asked to stay in Key Stage 1.

The warning signs are just as clear. Pressure to accept a booking you have already turned down. A different name on the phone every time. Rates that arrive with the payslip. Nobody available when a school day goes badly. And the classic: a flood of contact for the first fortnight, then silence.

Supply work has a reputation for being lonely, and with the wrong agency behind you it is. With the right one it is steady, varied work in schools you get to know, arranged by someone who knows your name and what you are good at.

We have worked with teachers and support staff across the North West for over 25 years, from our office in Handforth near Wilmslow. See the roles we are recruiting for on our jobs page, find out how registering works on our candidates page, meet the consultants you would actually be speaking to on meet the team, or start the conversation by registering your CV.