i'm gonna list down what criteria to check. u do the homework yourself.

It's about client protection level.
a forex broker can be 2k USD security deposit broker with no actual office and no audit, no segregated accounts, lax regulation, registered at some place called vanuatu, lousy support OR
a big corporation(even listed company) with hundreds of millions of assets with real offices across a few countries, audit, segregated account, strict regulation............
1. by assets size
find their reported assets, some are even listed companies like FXCM, OANDA

2. FOREX FINANCIAL license registration (https://www.100forexbrokers.com/fsa-regulated-brokers)
a. financial regulation by which countries,
some countries are very tight, audit, segregated account company with clients, (if bankrupt, liquidator cannot claim clients' money etc). like in US, owner sent to jail for defrauding customer
while some like Vanuatu is very lax. with less than 10k usd can register as broker. no audit, whatsoever.
great for scammer or whatever money game setup. owner and CEO wont go to jail.
usually the fishy ones love those lax regulation

b. how many countries are they registered at
oanda is registered at 6 countries like canada, US, England, singapore, australia, japan.

3. types of broker (main or white label)
some are just white label of the bigger forex broker. (white label is when u apply large volume transactions or large deposits in millions of USD), open another channel with big brokers (they dunt have to setup like normal big corporation forex broker.)
you will end up paying more commissions and again the client protection level is much lower

4. broker commissions structure. (if possible get them)
this is where u see the most fishy part. some "fishy brokers" offers up to 5times the normal market rate.
real brokers compete throught competitive spreads. if higher spreads means higher trading costs. multiply that over long period of time your profits will get eaten up. so real traders will avoid high spreads by default

money games brokers usually offers very high deposit bonus, high trading bonus for their introducer up to 5 to 10 times more.
the commission structure is exactly like pyramid schemes with 3layers of overriding.

even if the broker is legit, the broker will have to jack up your trading costs to cover it. end result is u get ripped off

5. currency spread (advertised vs real)
this u got to test it yourself. refer to point 4 & 6

6. trading side (this can be 5pages long)
- connection timed out, trade context is busy, go in at this price, but orders away by 3pips +spread (same goes to stop loss), sudden change of leverage prior to big announcement (if earlier sent u email for notification is ok)

7. audit
since they are regulated, they have audit. some are even listed companies, so undergo audit like KPMG

8. business address
- cross check with google, not some fishy virtual office share same address shared with a few more hundreds business registration. (a sign where u have more asset than your broker does)

9. bank in method
if go through local "agents" a big fucking NO  !! why ??
if i own the company, how would i know my agents will sapu my clients money, runaway, clients sue me, cause bad reputation ??

10. helpline / technical support
check their response time. call them up. 24/7 5days a week

11. test with small amount before u go big amount
at least u know how they operate